The XEN Wiki is propably the best starting point for further information.
Citrix owns the rights on XEN, but from time to time an "elder" release is released as open source. That open source version is being used by some Linux distributions.
The Hypervisor is loaded instead of a normal linux-kernel. After that a priviledged virtual system is loaded (called Dom0). That system is used to steer the Hypervisor - i.e. Network and Disk resources, whereas RAM and CPU are in control of the Hypervisior.
Other VMs are called "DomU". These can be fully XEN-awary with a special XEN-Kernel, able to run in PV (para-virtualized) mode. Other System can run in fully virtualized mode (HVM), but can use PV-drivers to access network and disk resources faster.
Currently known distributions that use the OpenSource XEN:
- RedHat 5 (and clones), Version 6 does not support Dom0 any more.
- Oracle VM (based on a RedHat Clone as well)
- SLES 10 and 11