@dom0 They are built from scratch using these components:
Upstream Xen Project PV driver sources, which includes contributions from Citrix, Vates, Invisible Things Lab among others, and where we are one of the maintainers;
The Rust guest agent, which is developed by us;
The installer, packaging and tooling is completely our own.
The main repo is here: https://github.com/xcp-ng/win-pv-drivers