Rackspace Hosting is in the midst of transforming itself from a dedicated hosting service provider into a public cloud player that also does substantial development on its cloud control freak like Amazon does, but is using the open source OpenStack controller and a mix of hypervisors* on its cloud. Specifically, Rackspace used open spource Xen as the hypervisor for the "legacy" public cloud for Linux-based Cloud Servers and now uses the commercial-grade XenServer from Citrix Systems for the new OpenStack powered public cloud for Linux-based Cloud Servers. XenServer has always underpinned for Windows-based Cloud Servers. But if you want an OpenStack-based private cloud to run in your data center but managed by Rackspace, this comes with the KVM hypervisor.