Xen is an old hypervisor. It supports x86, x86_64, PowerPC 970 ( apple processor) and Itanium. It has two versions. One commercial and other open source version. Its version comes in Essentials for XenServer , XenServer and Essentials for Hyper-V ( It is the management for Hyper-V . Its current version is XenServer 5.5. ( It too is free.) It supports both para-vert and full virtualisation with help of AMD-v and Intel _v technology. It supports live migration. It supports a plethora of OS therefore like Windows NT based , NetBSD , linux and solaris as guest and many as host like solaris, linux and netBSD. Citrix is quite an old player in the market. You can get its products for trial.

Rackspacecloud run on it.