Xens first public release was in 2003, becoming part of Novell SUSE 10 OS in 2005 (later also Red hat). In Oct 2007 Citrix acquired XenSource (the main maintainer of the Xen code) and released XenServer under the Citrix brand.
Version 5.6 was released in May 2010, 5.6SP2 in May 2011, 6.0 in Sep 2011, 6.1 in Sep 2012, 6.2 in June 2013, 6.5 in January 2015 and verion 7 in May 2016.