Why is my Xen DomU kernel using more power/heat when there is no work compared to the default kernel?

There is a lot more going on when you've got a VM running idle, as opposed to a system that isn't running a VM. On the other hand, it shouldn't be doing enough to heat up your "little room" (personal server?) significantly, and that heat certainly shouldn't be coming from the graphics cards.

My guess is that you've screwed up the kernel config in your custom build and it's disabled something that was probably quite important to your thermal management, but without knowing what you did, it's hard to point to a cause (there are hundreds of settings that could potentially contribute to this). Don't build kernels unless you know what you're doing; distro builds are almost always going to end up better than yours.