From Xen

XenSource, Inc. developed enterprise-grade virtualization solutions based on the industry's best server virtualization technology, the open source Xen hypervisor. Founded and run by the original Xen development team, XenSource products will allow enterprises to realize the TCO savings that result from server consolidation, increased utilization, and reduced complexity in the datacenter.

XenSource was acquired by Citrix in 2007.

The XenSource name has been retained as a string in the paths of several XenServer/XCP components, for example /etc/xensource, /opt/xensource, /var/log/xensource.log and so on.