We have recently converted one of Dual Xeon Quad core Win 2k3 server into Xen Vps, I must say performance is superior in Xen compared to physical server, even though Vps is configured with 2 cores only still it is outperforming by 50%.
There are several other windows Vps running smoothly without any stability issue.
FYI we are using XenServer 5.5.
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We've actually begun switching all of our MSVS (Microsoft Virtualization Services) boxes over to XenServer. Xen is a hell of a lot lighter on CPU and RAM than MSVS is, and we've had fewer issues since the switch.
The downside of all of this, of course, is the length of time it takes to convert a MSVS container to a Xen container. Everything else makes it worth it. Even the customers are happier.
Another benefit of Xen: It can handle up to 12 customers far better than MSVS could with 4 customers on the same exact machine. Actually, it could probably handle more than that on Xen... but I'm basing this off of our experience.
It doesn't come with a defacto control panel, HOWEVER, there IS an API for it. Check Xen's main site documentation for details on that.