i thought i heard kvm was winning, is there still a good reason to go with xen?
All are good choices.
XCP-NG + Xen Orchestra is a winning combo. If I had a large VM infra, that's the stack I would select. All open source too!
The tooling is really mature. Amazon used xen for ages. kvm wins for hardware passthrough and performance, but if your willing to sacrifice a little performance the advanced admin features are competitive.