Hi people.

Just wondering, I have plans to run some vm with Xen, now I don't know if I will use Centos 5.5 or Ubuntu as Host. I want to run as Guest:

FreeBSD7/8 and Ubuntu 9/10.

Do u have this 2 OS as guest on production with Centos or Ubuntu as Host?

Thanks!!!

Well, xen doesn't really have a "host" by design, just a system running privileged (called "dom0") with the ability to control the hypervisor and provide some resources like blockdevices, pci-devices etc. It is still a little problematic to get a reasonably recent kernel running with all xen-dom0-features needed. I suggest you read here:
http://wiki.xensource.com/xenwiki/XenDom0Kernels

You will probably at this time still use some forward-ported Xenlinux patches (officially released against 2.6.18) for dom0, as pvops-based dom0 isn't finished yet. According to the referenced page, Ubuntu's version seems to be buggy. I am running a debian system as dom0, but I built my own kernel using sources from SuSE, so if you want something running "out of the box", I'd suggest to try SuSE.

Guests will probably not give you any problems. Either they support Xen paravitualization, like newer Linux kernels etc -- or you get them running using hardware virtualization (one of my guests is a windows 2008 R2 x64 server).