Xen , the fastest growing open source virtualization project, is in the core of our own infrastructure. we've been early adopters , publishing our experiences from early on. For us paravirtualization and supporting VT is not a goal but a means to create a more flexible infrastructure. It helps us in achieving more with less.

From Xen.org

The Xen® hypervisor is the fastest and most secure infrastructure virtualization solution available today, supporting a wide range of guest operating systems including Windows®, Linux®, Solaris®, and various versions of the BSD operating systems. With Xen virtualization, a thin software layer known as the Xen hypervisor is inserted between the server's hardware and the operating system. This provides an abstraction layer that allows each physical server to run one or more "virtual servers", effectively decoupling the operating system and its applications from the underlying physical server.

The Xen hypervisor is a unique open source technology, developed collaboratively by the Xen community and engineers at over 20 of the most innovative data center solution vendors, including AMD, Cisco, Dell, HP, IBM, Intel, Mellanox, Network Appliance, Novell, Red Hat, SGI, Sun, Unisys, Veritas, Voltaire, and Citrix. Xen is licensed under the GNU General Public License (GPL2) and is available at no charge in both source and object format. Xen is, and always will be, open sourced, uniting the industry and the Xen ecosystem to speed the adoption of virtualization in the enterprise.

Xen is the Virtualization layer of choice for Citrix, Novell, Sun , Oracle and many more.