Xen lets KVM overtake
Xen made virtualisation a popular issue on Linux servers and seemed for a long time to be the open source virtualisation technology of choice. But the Xen developers lost that advantage by setting the wrong priorities. KVM was able to take over some key positions and, with a large number of promoters, is now in a better position.
You don't need to be a genius to debug a pf.conf firewall ruleset, you just need the guts to run tcpdump