Hyper-V (no overselling allowed and free with Windows 2008)
Hyper-V is great, however, one thing I do not like is the fact you cannot easily lower the amount of space allocated to a VHD. I tried numerous times and you just cannot do it. I was not using any sort of control panel (DNP), just manually working with the Hyper-V manager.
If you want true virtualization and not just process isolation then I would look at VMWare or Hyper-V. If you do not want 'true' virtualization then Virtuozzo is a big player in the Windows market.
Now, for linux, I very much love working with OpenVZ. Xen is also great, but I really love OpenVZ.
█ Daniel | Server Complete, LLC
█ Windows VPS // Dedicated Servers // Backup Services
█ Wholly owned hardware and self operated network (AS19531) in Jacksonville, FL
Hi. We are very happy with Xen so far. It's very flexible, and allow you to run a different kernel for each VM if you want to, which is not possible with VZ containers. Then you can allow your customers to use specific kernel for their OS if they feel like it, which is very good for driver compatibility (Asterisk, VPNs, etc. all need that). You will also be able to run NetBSD.
We have tried using windows for our own usage (both at home and remotely), and it works pretty well, especially with the GPL-PV drivers. I have a very limited experience in running it in production, but so far, so good. We just don't offer windows services because we don't feel we know enough to offer good support, and we have a passion for free software, but I really think some other people must be very happy with it.
What I regret a lot with Xen is that there's no support for 64 bits paravirtualized FreeBSD. In fact, I should say it the other way around. FreeBSD people did not yet release a 64 bits version of their os with Xen 3 support. We'd be so happy to be able to do that...
GPLHost:>_ open source hosting worldwide (I'm founder, CEO & official Debian Developer)
Servers & our leading control panel and our Xen VPS hosting, which are already included in Debian and Ubuntu
Available in: Kuala Lumpur, Singapore, Sydney, Seattle, Atlanta, Paris, London, Barcelona, Zurich, Israel