Are Cloud Operating Systems the Next Big Thing? gives the following answer:
Xen’s footprint in the Cloud: with AWS, Rackspace Public Cloud and many others running Xen, supporting Xen first makes sense.
Xen Paravirtualization provides a very simple and idealized interface for I/O to the guest. In contrast, the KVM VIRTIO interface looks pretty much like the underlying hardware. As a consequence, it is easier to port a language runtime to Xen.