Few tools around for making these tasks, none of them work for each thing, if you've your customized server with your own installation it's better to be scared by standard tools.
Here's want I'll do if I'm in your shoes:
- Install your debian server, customize it and install XEN as you normally do
- Create your virtual appliances inside XEN and prepare them for the boot
- Use clonezilla (http://clonezilla.org/) or similar tools to clone your machines, clonezilla and this category of tools doesn't care about virtual/physical machines, they simply clone your machine

Then you've your own cloned machine as virtual, some more thoughts about it:
- I personally use clonezilla because I'm happy with it, it's a suite of clone application, you can use whatever you want (personally I prefer UDP Cast, it simply rocks)
- If you don't want to install packages inside your existent or new Centos server you can simply boot these servers with clonezilla live cds, they simply need to clone them with UDPCast
- As the name says, UDPCast floods your disk in your net in UDP packets, use it in a private net (not the corporate one) if you want to leave, when you start flooding you'll run into troubles if you need the network for something else

Hope it helps

Try this:
http://helpdeskgeek.com/virtualization/convert-a-physical-server-to-xen-virtual-server-p2v/