About Xen
Xen is an open source virtual machine system. More information on Xen itself can be found at https://wiki.xensource.com/xenwiki/ and the Fedora Xen page.
Fedora is following the 3.0.x Xen line. Xen 3.0.0 was released in December of 2005 and is incompatible with guests using the previous Xen 2.0.x releases.