CentOS or OpenSolaris for Xen dom0?

You may also want to consider Citrix (XenSource) XenServer. It provides a self-contained dom0 similar to ESXi, and with an easier interface than Xen on CentOS.

Particularly if you're just starting to venture into Virtualization, XenServer can be a great choice. Particularly if you intend to run Windows on any of the guests, XenServer has been a smoother experience for me (compared to Xen in CentOS/RHEL 5 and to KVM on Ubuntu and CentoOS).

XenServer is also free, which makes it an excellent choice to use for Proof of Concepts (or even production, paid support is available if needed, too).

Advantages over the other two options:

• Self Contained, low administration overhead
• Small learning curve compared to manual virtualization setup
• Commercial support available if needed
• Better support for Windows guest VMs
• Good Documentation
• VM Management Console for managing VMs (XenCenter)
• Faster Development cycle (Much/most of the Xen development will be rolled out in new XenServer releases before they make it into a CentOS or OpenSolaris release)

Disadvantages:

• More of a black-box solution (less ability to customize/change dom0, although it is a Linux kernel and does allow console/ssh access, so you can make changes if you want)
• No ZFS support
• VM Management Console (XenCenter) is a Windows Application

I haven't experienced recently with Xen, however I believe ZFS alone should be sufficient to select OpenSolaris over CentOS as dom0. Having ZFS volumes as root disks for your domUs will allow you to have "for almost free" many snapshots of your environments, revert back when something goes wrong, clone previous versions should you need to compare them or want to duplicate a machine. Deduplication that was just integrated in the last OpenSolaris dev build should also improve this point.