I don't really understand. I thought XCP-NG was using the Xen hypervisor.
* 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.)