Yeah I'd check BIOS settings like @olivierlambert mentioned here.
But also, you mention pass through, this isn't pass through this is vGPU which is different (and only supported by some GPUs in specific), which allows you to have a single GPU that can run multiple VMs.
If you want passthrough you'll need to follow the docs here https://docs.xcp-ng.org/compute/
This will let you pass through the entire card, make sure to passthrough all devices that are part of the card (so HD audio and stuff too, everything that is under the same PCI address). I've done this with a few GPUs and it works great.