Quoting from the XenSource official announcement:
XenSource, Inc., the leader in infrastructure virtualization solutions based on the open source Xen™ hypervisor, today announced that their virtualization products have received IBM ServerProven certification.
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Solutions that are ServerProven represent applications that have been enabled for IBM Systems and have been verified through a customer experience to be installed and running in a customer location. To earn the IBM ServerProven certification, XenSource verified XenEnterprise can run on one or more IBM Systems in a real-world customer environment. IBM ServerProven certification is intended for IBM customers to be able to easily identify compatible, reliable solutions for their business-critical needs. XenSource products deliver bare-metal performance on a broad-range of IBM servers, including IBM BladeCenter and IBM System x, running on Windows or Linux.
Virtual Iron was in the process to obtain same status in June 2006.