There are a number of tools for provisioning physical and virtual servers, but I can't see a well documented solution for provisioning XenServer Debian guests, apart from using XenCenter itself.
With XenCenter guest VM provisioning, we could create a custom template from the specific version of Debian we want to use, specify the install media, and use a Preseed config file to automate the install options. Then once the VM is up and running, we can use some Config / State Management software to whip it in to shape.
However, this is for only deploying one VM at a time. What about provisioning multiple VMs at once, either via the command line, or ideally a web interface?