Mostly Linux, but also Windows when I had a VFIO passthrough setup. I don't think it's even possible to set it up with Virtualbox to have decent enough performance.
https://www.cyberus-technology.de/products/hypervisor (Don't mind the English, we are not native speakers. :)
We used to have support for Intel GVT-g GPU virtualization as well, which was more of a software solution. This doesn't work with modern Intel GPUs anymore.