The kernel inclusion of KVM makes me think about some virtualization adoption issues.
This much younger technology has made it into the kernel faster than Xen, is that important? Enabling virtualization support by using a vanilla Linux kernel configuration is important for grid computing mostly because it helps with resource provider adoption of virtualization capabilities which is probably the biggest impediment to VMs + grid computing right now.
But even though Xen has not made it into the mainline kernel (it’s been discussed for several years now, and has gone through some twists and turns), distribution packaging support for it is good because of the very strong demand for Xen.
Personally I think Xen is easy enough to install and deal with if you are already familiar with basic kernel configuration/installation. Its weakness in this area is when you need to diverge from the specific kernel versions the patches are geared to work against. It’s best to keep it as high up in the kernel dev chain as possible.
But overall, if you’ve already decided to support virtualization, the VMM’s features, cost, license, performance, and management options are much bigger factors than this. To me, it’s just a minor deployment detail — unless you have applications whose support contracts would be invalidated by switching kernels.
I would be interested to find out how much the lack of a native kernel option is really affecting administrators, especially if it is the one thing stopping them from running Xen — either because of support contract issues or “pain” issues (real or perceived).