The reason isn't so much that xen doesn't support it but more so that graphics cards are not very well intended for isolation. Correct me if i'm wrong but i don't think you're supposed to compute 1 thing on a gpu and expect another thing to not get access. In recent time Qubes has made progress on making a GPU VM where you can compute on a secondary GPU, but it only works for amd currently.

I use nvidia every day with qubes, but just for display output. I sometimes see memory leaks where it will draw screen buffers from booting alternate OSes on another drive.

yes that as well, I would doubt highly that the people at nvidia writing the driver for their pci-e desktop graphics cards (in my case was an NVS 510) are putting much consideration into things like xen. the card hardware and driver design is intended for single-user environments...