Xen debuted in 2003 and KVM in 2007. Xen has had a longer time in the market and hence will have a better market share. However, KVM has caught up fast and there are a few companies already using it in production. Of course, Qumranet were one (it's not Red Hat). A quick search throws these two results:
That goes a long way into saying the kind of efforts that will be put into kvm to make it as stable as possible for deploying into enterprises.
Fedora and Ubuntu already support KVM as the default hypervisor now.
That said, however, it's best to merge the Xen code upstream since having it out of tree while there are users out there doesn't seem like a good idea. One of the reasons distros quickly had to move to kvm as the default is because of the pain in maintaining out-of-tree patches and also maintaining different kernel versions just for Xen dom0 support.