Microsoft has been publishing Linux kernel patches for years now as it makes sure that Linux guests can run well within a Windows-hosted environment for virtualization. All of the core functionality for Linux guests hitting on Hyper-V have been in place for the mainline Linux kernel. Lately, they have been working on more improvements.
Some recent work includes the synthetic Linux GPU frame-buffer driver. That Linux FB driver was published originally last month but is now up to its third revision. Version 3 of the Hyper-V Synthetic Video Frame-Buffer Driver was published on Friday by a Microsoft engineer. "This is the driver for the Hyper-V Synthetic Video, which supports screen resolution up to Full HD 1920x1080 on Windows Server 2012 host, and 1600x1200 on Windows Server 2008 R2 or earlier. It also solves the double mouse cursor issue of the emulated video mode."
Also published on Friday were another chain of patches by Microsoft for Hyper-V. The set of six new patches enhances their memory balloon driver to add in support for memory hot-add. System memory to Linux guests is dynamically managed at run-time and now implemented for the Windows Dynamic Memory protocol, which is a combination of ballooning and hot-add for the dynamic balanacing of available memory across competing virtual machines.
It's nice continuing to see the continuing Linux kernel patches out of Microsoft that are destined for mainline. However, for those interested in cross-platform virtualization, I would much more strongly recommend VMware's products. I personally use VMware's products on a day-to-day basis on Linux and OS X to great success.
Oracle's VirtualBox platform generally works okay but their drivers are not mainline and are tainted crap. Their 3D guest support also isn't as good as the VMware work, with VMware employing many of the upstream Mesa/Gallium3D developers.
KVM virtualization works great if you're interested in Linux-based server virtualization, but for desktop virtualization their big shortcoming at the moment is no 3D/OpenGL guest support.