Hypervisors
"Virtualization architecture consisting of a thin software layer that runs directly on the hardware intercepting some, or all, operating system calls to hardware. Typically the hypervisor virtualizes CPU and memory while a single priviledged guest is assigned to manage and virtualize network and disk". (source: linuxvirtualization.com)

Xen
Open source virtualization software that is used to partition workstations and servers into separate virtual machines, each containing its own copy of an OS. Pronounced "zen," and developed at the University of Cambridge in the U.K., Xen is noted for its fast response and low overhead. Xen is a small, low-level "hypervisor," which is the first control software loaded when the computer starts up. Originally developed for x86 machines, support for the IA-64 (Itanium) and POWER5 (Mac) platforms followed. Xen gained wide acceptance in the Linux server market in the 2005 time frame. (source: PC Magazine)

Xen is an open-source software project that provides high-performance, resource-managed virtualization on the x86 processor architecture. It allows multiple operating system instances to run concurrently on a single physical computer. Xen manages the computer's hardware resources so they are shared effectively among the operating system instances, called domains. So what does all this mean? It means that by using Xen virtualization, you can run and use several different operating systems at the same time on a single computer. (source: the Linux Journal)