XenServer is a packaged product consisting of the Xen Project Hypervisor and the Xen Project Management API (XAPI) toolstack within a performance tuned CentOS distribution. It’s free and can be installed in just a few minutes; click here to download it: http://xenserver.org/open-source-virtualization-download.html.
Xen Orchestra (XO) is a simple but powerful web interface working out-of-the-box with XenServer, or any host with Xen and XAPI (the most advanced API for Xen). Take a look on the project website to learn more. Both of these tool are of course free software.