Looks like Microsoft releases the Linux drivers to enable a Linux kernel running as a guest in a Hyper-V hypervisor to run in ‘enlightened mode’, which sounds pretty much like Xen‘s PV drivers for Windows, providing better IO performance, under the GPLv2 (which is the same open-source license as the Linux kernel itself). Quoting the Hyper-V Architecture and Feature Overview:
Enlightened I/O is a specialized virtualization-aware implementation of high level communication protocols (such as SCSI) that utilize the VMBus directly, bypassing any device emulation layer. This makes the communication more efficient but requires an enlightened guest that is hypervisor and VMBus aware.
The drivers seem to be developed by Novell, so I guess the Boycott Novell guys will have some more coverage^Wrants soon (Update: can’t find the reference on this anymore, so this might be a false statement, sorry. Thanks for pointing out RubenV)