Xen

Xen is a virtual machine monitor (VMM) for x86-compatible computers. Xen can securely execute multiple virtual machines, each running its own OS, on a single physical system with close-to-native performance.

Xen Wiki

Xen (Wikipedia)

Xen FAQ

howto-install-windows-xp-vista-on-xen

A brief introduction to xen-tools (debian-administration.org)

Installing Xen 3.0 upon Debian Sarge