Xen is a hypervisor based virtualization technology originating at the University of Cambridge, nowadays developed largely by the company XenSource. Xen introduced the concept of paravirtualization, which allows for extremely high performance virtualization provided that the guest virtual machine runs a modified operating system kernel.
Xen also supports full virtualization (running unmodified operating systems) on processors with Intel VT or AMD-V technology.
Xen is available in many Linux distributions, however lhype and KVM appear to be making more progress at getting merged into the upstream kernel...
How to use Xen with a mobile network, for example on a laptop.
Lab 1: Xen Guest Installation (PDF) - Learn how to install RHEL3, RHEL4 and RHEL5 beta 2 and WinXP on Xen
Lab 2: Live Migration (PDF) - Want to move an executing workload from one node to another? Here's how.
Jailtime a site with many different Xen guest images.
Xen Remote Management Interfaces as summarized from a recent xen-devel post.