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: 
It's not the most capable, but if you're looking for a GUI with quick and dirty viewing and monitoring you can go with [virt-manager](http://virt-manager.et.redhat.com) from a gnome desktop. It's a redhat-sponsored project but is available in the Ubuntu desktop repos through aptitude/synaptic/whatever as well. 

We use it to with a bunch of KVM guests, it's also especially useful for VNC connectivity to the console when SSH isn't available.
Have you checked [RHEV](http://www.redhat.com/virtualization/rhev/) out?