That's because of the confusing names. Xen is not XenServer: * Xen is the hypervisor, the "upstream" used by both XenServer & XCP-ng * XenServer or XCP-ng is the platform, using Xen and various other things, to deliver a "turnkey" experience for server virtualization

Xen Project is part of the Linux foundation, and have various contributors, including AMD and Arm (for the embedded world) and others (including Vates, Citrix etc.)