KVM stands for the “Kernel Virtual Machine” and is included in the LInux-kernel as of version 2.6.20. The kvm wiki. This includes a list of supported KVM guests. A basic introduction While Red Hat and SUSE prefer Xen, Ubuntu chose in February 2008 to use KVM for virtualization. (Red Hat includes KVM in Fedora) Guides for using Ubuntu with KVM: A personal blog post on using KVM with Ubuntu Gutsy Gibbon. A how-to for Gutsy Gibbon (the latest Ubuntu release)

Unlike Redhat and SUSE, which use Xen, Ubuntu has decided to use the KVM Hypervisor

Xen is one of the most popular hypervisors, this article talks about using it in a common data center scenario.