ebook-convert does not require xvfb-run.
calibre's cli tools are truly headless -- as long as you have the x server libraries installed on linux, it "just works" without the need for virtual displays.
OTOH, I have no idea how complete Ubuntu for Windows 10 is, and supposedly not everything works yet.
Since calibre already has a native Windows build, why not stick with that for now? I doubt running calibre for linux under a beta linux emulator for Windows is going to receive any sort of proper support...