You can extract the drivers from the NetBSD kernel into exactly that, its called rump kernel http://www.rumpkernel.org http://www.rumpkernel.org and it can run

You can extract the drivers from the NetBSD kernel into exactly that, its called rump kernel http://www.rumpkernel.org http://www.rumpkernel.org and it can run on Xen among other places see https://github.com/rumpkernel/rumpuser-xen https://github.com/rumpkernel/rumpuser-xen or just in userspace under Linux say - it is getting easier to run any application on it, there is still a bit more work needed (thread support is not there yet). Working on it though, and it should get easier to build and run application code over the next few months. One advantage of using a BSD rather than Linux is licensing, as linking Linux kernel code into your application will probably force you to release as GPL. NetBSD also has an initial portability advantage, as you could always cross compile it.