Question: We are looking for an open-source or low cost virtual machine management control panel. We require it to be able to manage up to dozens of hypervisors and thousands of virtual machines. It should be either Xen or KVM, and have an API to hook into, to create, delete, modify virtual machines. Also, it should be able to provide reports/graphs of each virtual machines CPU, Memory, and used network throughput.

We know about Citrix XenServer, and its awesome, but it is cost prohibitive. Any other options available?

Answer: 
You probably want [libvirt](http://www.libvirt.org/).

Supports Xen, KVM, Virtualbox, VMWare, etc. Has a stable API, multiple language bindings.

I've never written anything that uses libvirt, but I use the libvirt virtual machine manager (GUI); and virsh (CLI), which can be scripted.
Have you checked [RHEV](http://www.redhat.com/virtualization/rhev/) out?