Xen to KVM windows guest migration

That benchmark is only comparing the speed of the native OS to a single guest OS. It is hardly a real-world test. I don't think I would put much weight on it. Most of the KVM camp argues that Xen requires too many interrupts and hops between kernel and user space, but from most of the more real-world benchmarks that I've seen that hasn't really been realized and Xen seems to be a bit faster than KVM.

Sorry I don't have a link to back that up handy. But I will say that KVM is improving fast and seems to be catching up on feature set and stability quickly.

As to which approach is better. The Xen camp will argue that a true light-weight hypervisor is required virtualization to be secure and fast. Xen is also starting to be supported in firmware by some vendors which is also nice. The KVM camp will argue that KVM is simpler and that Linux is capable of being a good hypervisor.

In the end it's still unclear which direction will ultimately win. Xen certainly has a head start and already has a nice market share. But it's not in the mainline kernel, yet. Hopefully that will change soon and there has certainly been a lot of talk about this on the kernel list in the past few months. Red Hat is in the KVM camp now and will be pushing it as the virtualization platform of choice. Red Hat Linux 5.4 which is coming out shortly will be the first to include it. So that will likely attract shops that haven't rolled out or committed to a virtualization platform yet.

As far as tools go, both Xen and KVM use libvirt and QEMU and the tools associated with them. So they share many of the same tools such as virt-manager.

We use Xen at work, and it works well for us. But I've been looking into KVM due to some USB forwarding and PCI passthrough issues I've been unable to resolve with Xen. I'm not sure KVM is any better at this, but I guess I'll find out once I try it. One thing I have noticed in researching my USB issues is that KVM's documentation is more assessable and organized compared to Xen's. But there is no perfect virtualization platform so you'll need to figure out what makes sense for you.

I would go with XEN. Two main reasons. First, you can get it installed and configured painlessly by installing OpenSUSE 11, and you get a GUI to administer it as well. But all the technical stuff is still there at the shell prompt so that you can learn the innards of XEN as and when you want to. Second reason is that XEN is the top choice of companies who provide VPS hosting, and if you think that you might someday want to use your knowledge to get a job, XEN is a better bet. If you are thinking of the career enhancing effect, make sure to look at the innards of XEN, the network virtualization on Linux (which is not part of XEN but useful for building testbeds of routed VMs) and the XEN api. There are various XEN management packages which take care of management for you, but in specialised scenarios, it is always good to be able to write some scripts (probably in Python) to do your own management/monitoring/reporting.

XEN meets all your bullet points and has been around for longer which is why it is the choice of hosting folks.

Note that you might want to also check out OpenVZ which is completely different from XEN but also gets used a lot by VPS hosting companies. Since OpenVZ uses a container model of virtualisation, you can actually run OpenVZ inside a XEN guest.

While you are doing this, think about your disk storage. You probably want volume management, which is also included in OpenSUSE with a nice GUI, so that you can assign hard disk partitions or volumes to the guest OS. For instance, you could even set it up so that a Windows partition can be booted from, or used by the XEN guest.