Xen came before KVM, but ultimately Redhat played a heavy hand to swing much of the market
but with Citrix it managed to survive (so far).
XCP-ng is a recent open source spin-off, which attempts to gather a larger community.
Their XOSAN storage is aimed to deliver a HCI solution somewhat like Gluster. But designed
only as a companion to XCP-ng, may be easier to maintain and faster. v2 is in the works
and not backward compatible to v1, so you may want to start with other alternatives, of
which there are a few.
https://xcp-ng.org/docs/
https://xen-orchestra.com/#!/xo-home