We use NOC-PS. It's actually a great peice of software, well worth the money. It costs just 1Euro /yr per server. You can't really go wrong!
Maybe there are better or free options out there. But for us, this is just so easy to use!
We used to do manual installs on site. Couple of remote installs later and hey we've got our money's worth right there.
Installation is a breeze. Just download the file and load it to your VM. Done! Its practically all set up and ready to go as is. It does so much more than remote install too, like traffic graphing. Excellent stuff!
Actually, NOC-PS offer a number of features that are 'unsupported'. Meaning, they include it in the software, but provide no support or warranties for it. So if that part doesn't work, tough. You're paying for the provisioning part. Everything else is just.. being kind :-).
We use VMWare for the virtualization. But as far as I know, they have a special image for XEN servers as well as ISOs for other virtualizers out there. It's pretty extensive really, considering it runs on FREE systems. If you have a dedicated server available, which I assume you should since you will be provisioning them with this software, just bung on VMWare and put it on! It was mucho simple for us.