<s>
Xen	O
(	O
pronounced	O
)	O
is	O
a	O
free	B-License
and	I-License
open-source	I-License
type-1	O
hypervisor	B-Operating_System
,	O
providing	O
services	O
that	O
allow	O
multiple	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
to	O
execute	O
on	O
the	O
same	O
computer	B-Architecture
hardware	I-Architecture
concurrently	O
.	O
</s>
<s>
originally	O
developed	O
by	O
the	O
University	O
of	O
Cambridge	O
Computer	O
Laboratory	O
and	O
is	O
now	O
being	O
developed	O
by	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
with	O
support	O
from	O
Intel	O
,	O
Citrix	B-Application
,	O
Arm	B-Architecture
Ltd	O
,	O
Huawei	O
,	O
AWS	B-Application
,	O
Alibaba	B-General_Concept
Cloud	I-General_Concept
,	O
AMD	O
,	O
Bitdefender	B-Application
and	O
epam	B-Architecture
.	O
</s>
<s>
The	O
Xen	B-Application
Project	I-Application
community	O
develops	O
and	O
maintains	O
Xen	B-Application
Project	I-Application
as	O
free	B-License
and	I-License
open-source	I-License
software	I-License
,	O
subject	O
to	O
the	O
requirements	O
of	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
(	O
GPL	B-License
)	O
,	O
version	O
2	O
.	O
</s>
<s>
Xen	B-Application
Project	I-Application
is	O
currently	O
available	O
for	O
the	O
IA-32	B-Device
,	O
x86-64	B-Device
and	O
ARM	B-Architecture
instruction	I-Architecture
sets	I-Architecture
.	O
</s>
<s>
Xen	B-Application
Project	I-Application
runs	O
in	O
a	O
more	O
privileged	O
CPU	O
state	O
than	O
any	O
other	O
software	O
on	O
the	O
machine	O
,	O
except	O
for	O
Firmware	B-Application
.	O
</s>
<s>
Responsibilities	O
of	O
the	O
hypervisor	B-Operating_System
include	O
memory	O
management	O
and	O
CPU	O
scheduling	O
of	O
all	O
virtual	O
machines	O
(	O
"	O
domains	O
"	O
)	O
,	O
and	O
for	O
launching	O
the	O
most	O
privileged	O
domain	O
(	O
"	O
dom0	O
"	O
)	O
-	O
the	O
only	O
virtual	O
machine	O
which	O
by	O
default	O
has	O
direct	O
access	O
to	O
hardware	B-Architecture
.	O
</s>
<s>
From	O
the	O
dom0	O
the	O
hypervisor	B-Operating_System
can	O
be	O
managed	O
and	O
unprivileged	O
domains	O
(	O
"	O
domU	O
"	O
)	O
can	O
be	O
launched	O
.	O
</s>
<s>
The	O
dom0	O
domain	O
is	O
typically	O
a	O
version	O
of	O
Linux	B-Application
or	O
BSD	B-Device
.	O
</s>
<s>
User	O
domains	O
may	O
either	O
be	O
traditional	O
operating	B-General_Concept
systems	I-General_Concept
,	O
such	O
as	O
Microsoft	B-Application
Windows	I-Application
under	O
which	O
privileged	O
instructions	O
are	O
provided	O
by	O
hardware	B-General_Concept
virtualization	I-General_Concept
instructions	O
(	O
if	O
the	O
host	O
processor	O
supports	O
x86	B-General_Concept
virtualization	I-General_Concept
,	O
e.g.	O
,	O
Intel	O
VT-x	O
and	O
AMD-V	O
)	O
,	O
or	O
paravirtualized	O
operating	B-General_Concept
systems	I-General_Concept
whereby	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
aware	O
that	O
it	O
is	O
running	O
inside	O
a	O
virtual	O
machine	O
,	O
and	O
so	O
makes	O
hypercalls	B-Operating_System
directly	O
,	O
rather	O
than	O
issuing	O
privileged	O
instructions	O
.	O
</s>
<s>
Xen	B-Application
Project	I-Application
boots	O
from	O
a	O
bootloader	B-Application
such	O
as	O
GNU	B-Application
GRUB	I-Application
,	O
and	O
then	O
usually	O
loads	O
a	O
paravirtualized	O
host	O
operating	B-General_Concept
system	I-General_Concept
into	O
the	O
host	O
domain	O
(	O
dom0	O
)	O
.	O
</s>
<s>
To	O
support	O
embedded	O
systems	O
such	O
as	O
smartphone/	O
IoT	O
with	O
relatively	O
scarce	O
hardware	B-Architecture
computing	O
resources	O
,	O
the	O
Secure	O
Xen	O
ARM	B-Architecture
architecture	I-Architecture
on	O
an	O
ARM	B-Architecture
CPU	O
was	O
exhibited	O
at	O
Xen	O
Summit	O
on	O
April	O
17	O
,	O
2007	O
,	O
held	O
in	O
IBM	O
TJ	O
Watson	O
.	O
</s>
<s>
The	O
first	O
public	O
release	O
of	O
Secure	O
Xen	O
ARM	B-Architecture
source	O
code	O
was	O
made	O
at	O
Xen	O
Summit	O
on	O
June	O
24	O
,	O
2008	O
by	O
Sang-bum	O
Suh	O
,	O
a	O
Cambridge	O
alumnus	O
,	O
in	O
Samsung	O
Electronics	O
.	O
</s>
<s>
On	O
October	O
22	O
,	O
2007	O
,	O
Citrix	B-Application
Systems	I-Application
completed	O
its	O
acquisition	O
of	O
XenSource	O
,	O
and	O
the	O
Xen	B-Application
Project	I-Application
moved	O
to	O
the	O
xen.org	O
domain	O
.	O
</s>
<s>
This	O
move	O
had	O
started	O
some	O
time	O
previously	O
,	O
and	O
made	O
public	O
the	O
existence	O
of	O
the	O
Xen	B-Application
Project	I-Application
Advisory	O
Board	O
(	O
Xen	O
AB	O
)	O
,	O
which	O
had	O
members	O
from	O
Citrix	B-Application
,	O
IBM	O
,	O
Intel	O
,	O
Hewlett-Packard	O
,	O
Novell	O
,	O
Red	O
Hat	O
,	O
Sun	O
Microsystems	O
and	O
Oracle	B-Application
.	O
</s>
<s>
The	O
Xen	O
Advisory	O
Board	O
advises	O
the	O
Xen	B-Application
Project	I-Application
leader	O
and	O
is	O
responsible	O
for	O
the	O
Xen	O
trademark	O
,	O
which	O
Citrix	B-Application
has	O
freely	O
licensed	O
to	O
all	O
vendors	O
and	O
projects	O
that	O
implement	O
the	O
Xen	O
hypervisor	B-Operating_System
.	O
</s>
<s>
Citrix	B-Application
also	O
used	O
the	O
Xen	O
brand	O
itself	O
for	O
some	O
proprietary	O
products	O
unrelated	O
to	O
Xen	O
,	O
including	O
XenApp	B-General_Concept
and	O
XenDesktop	B-General_Concept
.	O
</s>
<s>
On	O
April	O
15	O
,	O
2013	O
,	O
it	O
was	O
announced	O
that	O
the	O
Xen	B-Application
Project	I-Application
was	O
moved	O
under	O
the	O
auspices	O
of	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
as	O
a	O
Collaborative	O
Project	O
.	O
</s>
<s>
The	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
launched	O
a	O
new	O
trademark	O
for	O
"	O
Xen	B-Application
Project	I-Application
"	O
to	O
differentiate	O
the	O
project	O
from	O
any	O
commercial	O
use	O
of	O
the	O
older	O
"	O
Xen	O
"	O
trademark	O
.	O
</s>
<s>
Project	O
members	O
at	O
the	O
time	O
of	O
the	O
announcement	O
included	O
:	O
Amazon	O
,	O
AMD	O
,	O
Bromium	O
,	O
CA	O
Technologies	O
,	O
Calxeda	O
,	O
Cisco	O
,	O
Citrix	B-Application
,	O
Google	O
,	O
Intel	O
,	O
Oracle	B-Application
,	O
Samsung	O
,	O
and	O
Verizon	O
.	O
</s>
<s>
The	O
Xen	B-Application
project	I-Application
itself	O
is	O
self-governing	O
.	O
</s>
<s>
Since	O
version	O
3.0	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
Xen	O
support	O
for	O
dom0	O
and	O
domU	O
exists	O
in	O
the	O
mainline	O
kernel	B-Operating_System
.	O
</s>
<s>
Support	O
for	O
the	O
Intel	O
IA-64	B-General_Concept
architecture	O
.	O
</s>
<s>
Support	O
for	O
the	O
PowerPC	B-Architecture
architecture	O
.	O
</s>
<s>
Xen	O
ARM	B-Architecture
hypervisor	B-Operating_System
source	O
code	O
released	O
for	O
ARM	B-Architecture
CPU	O
support	O
3.4	O
Contains	O
a	O
first	O
version	O
of	O
the	O
"	O
Xen	O
Client	O
Initiative	O
"	O
,	O
shortly	O
XCI	O
.	O
</s>
<s>
4.0	O
Makes	O
it	O
possible	O
to	O
use	O
a	O
dom0	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
which	O
has	O
been	O
implemented	O
by	O
using	O
PVOps	O
.	O
</s>
<s>
A	O
Linux	B-Operating_System
kernel	I-Operating_System
of	O
version	O
2.6.31	O
has	O
been	O
modified	O
for	O
this	O
purpose	O
,	O
because	O
the	O
official	O
Linux	B-Operating_System
kernel	I-Operating_System
actually	O
does	O
not	O
support	O
the	O
usage	O
as	O
dom0	O
kernel	B-Operating_System
(	O
date	O
July	O
2010	O
)	O
.	O
</s>
<s>
Linux	B-Operating_System
kernel	I-Operating_System
v2.6.37	O
and	O
onward	O
support	O
usage	O
as	O
dom0	O
kernel	B-Operating_System
.	O
</s>
<s>
4.3	O
Experimental	O
ARM	B-Architecture
support	O
.	O
</s>
<s>
Support	O
for	O
Open	B-Language
vSwitch	I-Language
.	O
</s>
<s>
4.4	O
Solid	O
libvirt	O
support	O
for	O
libxl	O
,	O
new	O
scalable	O
event	O
channel	O
interface	O
,	O
hypervisor	B-Operating_System
ABI	B-Operating_System
for	O
ARM	B-Architecture
declared	O
stable	O
,	O
Nested	O
Virtualization	O
on	O
Intel	O
hardware	B-Architecture
.	O
</s>
<s>
Internet	O
hosting	O
service	O
companies	O
use	O
hypervisors	B-Operating_System
to	O
provide	O
virtual	B-General_Concept
private	I-General_Concept
servers	I-General_Concept
.	O
</s>
<s>
Amazon	B-General_Concept
EC2	I-General_Concept
(	O
since	O
August	O
2006	O
)	O
,	O
IBM	O
SoftLayer	B-General_Concept
,	O
Liquid	O
Web	O
,	O
Fujitsu	B-General_Concept
Global	I-General_Concept
Cloud	I-General_Concept
Platform	I-General_Concept
,	O
Linode	B-General_Concept
,	O
OrionVM	B-General_Concept
and	O
Rackspace	B-Application
Cloud	I-Application
use	O
Xen	O
as	O
the	O
primary	O
VM	O
hypervisor	B-Operating_System
for	O
their	O
product	O
offerings	O
.	O
</s>
<s>
Virtual	B-Operating_System
machine	I-Operating_System
monitors	I-Operating_System
(	O
also	O
known	O
as	O
hypervisors	B-Operating_System
)	O
also	O
often	O
operate	O
on	O
mainframes	B-Architecture
and	O
large	O
servers	O
running	O
IBM	O
,	O
HP	O
,	O
and	O
other	O
systems	O
.	O
</s>
<s>
Server	B-General_Concept
virtualization	I-General_Concept
can	O
provide	O
benefits	O
such	O
as	O
:	O
</s>
<s>
Xen	O
's	O
support	O
for	O
virtual	O
machine	O
live	O
migration	O
from	O
one	O
host	O
to	O
another	O
allows	O
load	B-Application
balancing	I-Application
and	O
the	O
avoidance	O
of	O
downtime	O
.	O
</s>
<s>
Virtualization	O
also	O
has	O
benefits	O
when	O
working	O
on	O
development	O
(	O
including	O
the	O
development	O
of	O
operating	B-General_Concept
systems	I-General_Concept
)	O
:	O
running	O
the	O
new	O
system	O
as	O
a	O
guest	O
avoids	O
the	O
need	O
to	O
reboot	O
the	O
physical	O
computer	O
whenever	O
a	O
bug	O
occurs	O
.	O
</s>
<s>
Finally	O
,	O
hardware	B-Architecture
appliance	O
vendors	O
may	O
decide	O
to	O
ship	O
their	O
appliance	O
running	O
several	O
guest	O
systems	O
,	O
so	O
as	O
to	O
be	O
able	O
to	O
execute	O
various	O
pieces	O
of	O
software	O
that	O
require	O
different	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Xen	O
offers	O
five	O
approaches	O
to	O
running	O
the	O
guest	O
operating	B-General_Concept
system	I-General_Concept
:	O
</s>
<s>
HVM	O
(	O
hardware	B-General_Concept
virtual	I-General_Concept
machine	I-General_Concept
)	O
:	O
Fully	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
with	O
emulated	O
devices	O
.	O
</s>
<s>
HVM	O
with	O
PV	O
drivers	O
:	O
Fully	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
with	O
PV	O
drivers	O
for	O
IO	O
devices	O
.	O
</s>
<s>
PVHVM	O
(	O
paravirtualization	O
with	O
hardware	B-General_Concept
virtualization	I-General_Concept
)	O
:	O
PV	O
supported	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
with	O
PV	O
drivers	O
for	O
IO	O
devices	O
.	O
</s>
<s>
PVH	O
(	O
PV	O
in	O
an	O
HVM	O
container	O
)	O
:	O
Fully	O
paravirtualized	O
Guest	O
accelerated	O
by	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
where	O
available	O
.	O
</s>
<s>
Xen	O
provides	O
a	O
form	O
of	O
virtualization	O
known	O
as	O
paravirtualization	O
,	O
in	O
which	O
guests	O
run	O
a	O
modified	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
guests	O
are	O
modified	O
to	O
use	O
a	O
special	O
hypercall	B-Operating_System
ABI	B-Operating_System
,	O
instead	O
of	O
certain	O
architectural	O
features	O
.	O
</s>
<s>
Paravirtualization	O
avoids	O
the	O
need	O
to	O
emulate	O
a	O
full	O
set	O
of	O
hardware	B-Architecture
and	O
firmware	B-Application
services	O
,	O
which	O
makes	O
a	O
PV	O
system	O
simpler	O
to	O
manage	O
and	O
reduces	O
the	O
attack	O
surface	O
exposed	O
to	O
potentially	O
malicious	O
guests	O
.	O
</s>
<s>
On	O
32-bit	B-Device
x86	I-Device
,	O
the	O
Xen	O
host	O
kernel	B-Operating_System
code	O
runs	O
in	O
Ring	B-Operating_System
0	I-Operating_System
,	O
while	O
the	O
hosted	O
domains	O
run	O
in	O
Ring	B-Operating_System
1	I-Operating_System
(	O
kernel	B-Operating_System
)	O
and	O
Ring	B-Operating_System
3	I-Operating_System
(	O
applications	O
)	O
.	O
</s>
<s>
CPUs	O
that	O
support	O
virtualization	O
make	O
it	O
possible	O
to	O
run	O
unmodified	O
guests	O
,	O
including	O
proprietary	O
operating	B-General_Concept
systems	I-General_Concept
(	O
such	O
as	O
Microsoft	B-Application
Windows	I-Application
)	O
.	O
</s>
<s>
This	O
is	O
known	O
as	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
,	O
however	O
,	O
in	O
Xen	O
this	O
is	O
known	O
as	O
hardware	B-General_Concept
virtual	I-General_Concept
machine	I-General_Concept
(	O
HVM	O
)	O
.	O
</s>
<s>
HVM	O
extensions	O
provide	O
additional	O
execution	O
modes	O
,	O
with	O
an	O
explicit	O
distinction	O
between	O
the	O
most-privileged	O
modes	O
used	O
by	O
the	O
hypervisor	B-Operating_System
with	O
access	O
to	O
the	O
real	O
hardware	B-Architecture
(	O
called	O
"	O
root	O
mode	O
"	O
in	O
x86	O
)	O
and	O
the	O
less-privileged	O
modes	O
used	O
by	O
guest	O
kernels	B-Operating_System
and	O
applications	O
with	O
"	O
hardware	B-Architecture
"	O
accesses	O
under	O
complete	O
control	O
of	O
the	O
hypervisor	B-Operating_System
(	O
in	O
x86	O
,	O
known	O
as	O
"	O
non-root	O
mode	O
"	O
;	O
both	O
root	O
and	O
non-root	O
mode	O
have	O
Rings	O
0	O
–	O
3	O
)	O
.	O
</s>
<s>
Use	O
of	O
ARM	B-Architecture
v7A	O
and	O
v8A	O
virtualization	O
extensions	O
came	O
with	O
Xen	O
4.3	O
.	O
</s>
<s>
HVM	O
extensions	O
also	O
often	O
offer	O
new	O
instructions	O
to	O
allow	O
direct	O
calls	O
by	O
a	O
paravirtualized	O
guest/driver	O
into	O
the	O
hypervisor	B-Operating_System
,	O
typically	O
used	O
for	O
I/O	O
or	O
other	O
operations	O
needing	O
high	O
performance	O
.	O
</s>
<s>
In	O
current	O
versions	O
of	O
Xen	O
(	O
up	O
to	O
4.2	O
)	O
only	O
fully	O
virtualized	O
HVM	O
guests	O
can	O
make	O
use	O
of	O
hardware	B-Architecture
facilities	O
for	O
multiple	O
independent	O
levels	O
of	O
memory	O
protection	O
and	O
paging	O
.	O
</s>
<s>
Xen	O
HVM	O
has	O
device	O
emulation	O
based	O
on	O
the	O
QEMU	B-Application
project	O
to	O
provide	O
I/O	O
virtualization	O
to	O
the	O
virtual	O
machines	O
.	O
</s>
<s>
The	O
system	O
emulates	O
hardware	B-Architecture
via	O
a	O
patched	O
QEMU	B-Application
"	O
device	O
manager	O
"	O
(	O
qemu-dm	O
)	O
daemon	O
running	O
as	O
a	O
backend	O
in	O
dom0	O
.	O
</s>
<s>
In	O
a	O
performance-critical	O
environment	O
,	O
PV-on-HVM	O
disk	O
and	O
network	O
drivers	O
are	O
used	O
during	O
the	O
normal	O
guest	O
operation	O
,	O
so	O
that	O
the	O
emulated	O
PC	B-Architecture
hardware	I-Architecture
is	O
mostly	O
used	O
for	O
booting	O
.	O
</s>
<s>
The	O
Xen	O
hypervisor	B-Operating_System
has	O
been	O
ported	O
to	O
a	O
number	O
of	O
processor	O
families	O
:	O
</s>
<s>
ARM	B-Architecture
:	O
previously	O
supported	O
under	O
the	O
XenARM	O
project	O
for	O
older	O
versions	O
of	O
ARM	B-Architecture
without	O
virtualization	O
extensions	O
,	O
such	O
as	O
the	O
Cortex-A9	O
.	O
</s>
<s>
Currently	O
supported	O
since	O
Xen	O
4.3	O
for	O
newer	O
versions	O
of	O
the	O
ARM	B-Architecture
with	O
virtualization	O
extensions	O
,	O
such	O
as	O
the	O
Cortex-A15	O
.	O
</s>
<s>
Xen	O
can	O
be	O
shipped	O
in	O
a	O
dedicated	O
virtualization	O
platform	O
,	O
such	O
as	O
XCP-ng	B-Application
or	O
Citrix	B-Application
Hypervisor	B-Operating_System
(	O
formerly	O
Citrix	B-Application
XenServer	O
,	O
and	O
before	O
that	O
XenSource	O
's	O
XenEnterprise	O
)	O
.	O
</s>
<s>
Alternatively	O
,	O
Xen	O
is	O
distributed	O
as	O
an	O
optional	O
configuration	O
of	O
many	O
standard	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Alpine	O
Linux	B-Application
offers	O
a	O
minimal	O
dom0	O
system	O
(	O
Busybox	B-Application
,	O
UClibc	B-Language
)	O
that	O
can	O
be	O
run	O
from	O
removable	O
media	O
,	O
like	O
USB	O
sticks	O
.	O
</s>
<s>
Arch	O
Linux	B-Application
provides	O
the	O
necessary	O
packages	O
with	O
detailed	O
setup	O
instructions	O
on	O
their	O
Wiki	O
.	O
</s>
<s>
Debian	O
Linux	B-Application
(	O
since	O
version	O
4.0	O
"	O
etch	O
"	O
)	O
and	O
many	O
of	O
its	O
derivatives	O
;	O
</s>
<s>
FreeBSD	B-Operating_System
11	O
includes	O
experimental	O
host	O
support	O
.	O
</s>
<s>
Gentoo	B-Application
has	O
the	O
necessary	O
packages	O
available	O
to	O
support	O
Xen	O
,	O
along	O
with	O
instructions	O
on	O
their	O
Wiki	O
.	O
</s>
<s>
NetBSD	B-Device
can	O
function	O
as	O
domU	O
and	O
dom0	O
.	O
</s>
<s>
OpenSolaris-based	O
distributions	O
can	O
function	O
as	O
dom0	O
and	O
domU	O
from	O
Nevada	O
build	O
75	O
onwards	O
.	O
</s>
<s>
openSUSE	B-Operating_System
10.x	O
to	O
12.x	O
:	O
only	O
64-bit	O
hosts	O
are	O
supported	O
since	O
12.1	O
;	O
</s>
<s>
SUSE	O
Linux	B-Application
Enterprise	O
Server	O
(	O
since	O
version	O
10	O
)	O
;	O
</s>
<s>
Solaris	B-Application
(	O
since	O
2013	O
with	O
Oracle	B-Application
VM	I-Application
Server	I-Application
for	I-Application
x86	I-Application
,	O
before	O
with	O
Sun	O
xVM	O
)	O
;	O
</s>
<s>
Ubuntu	B-Operating_System
(	O
since	O
8.04	O
"	O
Hardy	O
Heron	O
"	O
,	O
but	O
no	O
dom0-capable	O
kernel	B-Operating_System
in	O
8.10	O
"	O
Intrepid	O
Ibex	O
"	O
until	O
12.04	O
"	O
Precise	O
Pangolin	O
"	O
.	O
)	O
</s>
<s>
Guest	O
systems	O
can	O
run	O
fully	O
virtualized	O
(	O
which	O
requires	O
hardware	B-Architecture
support	O
)	O
,	O
paravirtualized	O
(	O
which	O
requires	O
a	O
modified	O
guest	O
operating	B-General_Concept
system	I-General_Concept
)	O
,	O
or	O
fully	O
virtualized	O
with	O
paravirtualized	O
drivers	O
(	O
PVHVM	O
)	O
.	O
</s>
<s>
Most	O
operating	B-General_Concept
systems	I-General_Concept
which	O
can	O
run	O
on	O
PCs	O
can	O
run	O
as	O
a	O
Xen	O
HVM	O
guest	O
.	O
</s>
<s>
FreeBSD	B-Operating_System
in	O
32-bit	O
,	O
or	O
64-bit	O
through	O
PVHVM	O
;	O
</s>
<s>
OpenBSD	B-Operating_System
,	O
through	O
PVHVM	O
;	O
</s>
<s>
Xen	O
version	O
3.0	O
introduced	O
the	O
capability	O
to	O
run	O
Microsoft	B-Application
Windows	I-Application
as	O
a	O
guest	O
operating	B-General_Concept
system	I-General_Concept
unmodified	O
if	O
the	O
host	B-Operating_System
machine	I-Operating_System
's	O
processor	O
supports	O
hardware	B-General_Concept
virtualization	I-General_Concept
provided	O
by	O
Intel	O
VT-x	O
(	O
formerly	O
codenamed	O
Vanderpool	O
)	O
or	O
AMD-V	O
(	O
formerly	O
codenamed	O
Pacifica	O
)	O
.	O
</s>
<s>
During	O
the	O
development	O
of	O
Xen	O
1.x	O
,	O
Microsoft	O
Research	O
,	O
along	O
with	O
the	O
University	O
of	O
Cambridge	O
Operating	B-General_Concept
System	I-General_Concept
group	O
,	O
developed	O
a	O
port	O
of	O
Windows	B-Application
XP	I-Application
to	O
Xen	O
—	O
made	O
possible	O
by	O
Microsoft	O
's	O
Academic	O
Licensing	O
Program	O
.	O
</s>
<s>
Without	O
these	O
drivers	O
all	O
disk	O
and	O
network	O
traffic	O
has	O
to	O
be	O
processed	O
through	O
QEMU-DM	O
.	O
</s>
<s>
Subsequently	O
,	O
Citrix	B-Application
has	O
released	O
under	O
a	O
BSD	B-Device
license	O
(	O
and	O
continues	O
to	O
maintain	O
)	O
PV	O
drivers	O
for	O
Windows	O
.	O
</s>
<s>
Novell	O
's	O
PlateSpin	O
Orchestrate	O
also	O
manages	O
Xen	O
virtual	O
machines	O
for	O
Xen	O
shipping	O
in	O
SUSE	O
Linux	B-Application
Enterprise	O
Server	O
.	O
</s>
<s>
Crucible	O
(	O
hypervisor	B-Operating_System
)	O
by	O
Star	O
Lab	O
Corp	O
.	O
</s>
<s>
The	O
Xen	O
hypervisor	B-Operating_System
is	O
covered	O
by	O
the	O
GNU	B-License
General	I-License
Public	I-License
Licence	I-License
,	O
so	O
all	O
of	O
these	O
versions	O
contain	O
a	O
core	O
of	O
free	O
software	O
with	O
source	O
code	O
.	O
</s>
