Microsoft has thrown down the gauntlet to VMware by launching the lastest version of its Hyper-V hypervisor as a free download, cheekily offering the software to coincide with the first day of the VMworld show in San Francisco.

The hypervisor, which is available as a standalone download as well as being part of Windows Server 2008.

The move follows a week of bust-ups between Microsoft and VMware. First of all, Microsoft ceased to be a sponsor of the VMworld show following a claim by the software giant that VMware had laid down a series of unacceptable conditions. Citrix also joined in the fun by pulling out as a sponsor too.

Then, there was an entertaining war in the blogosphere as Microsoft blogger Jeff Woolsey slated the footprint size of the ESXi hypervisor, pointing out that "VMware touts ESXi as a 32 MB hypervisor, yet the download is over 200 MB. So, are we too assume that the other 170+ MB doesn't count? "

Woolsey then went to on compare the patch records of the two hypervisors, pointing out that while tHyper-V Server 2008: had 82MB of patches between 30 June 2008 and 30 June 2009, ESXi had 2.7GB's worth of patches. Although, Microsoft also admitted to twice as many patches as Vmware.

This brought a heated response from VMware blogger Eric Horschman pointing out that the actual size of ESXi was 59.3MB - somewhat less than the 70MB figure we've publicly stated and that the Hyper-V footprint was far larger, before going to describe how Hyper-V would remain an integral part of Windows, while VMware had decoupled the hypervisor from the operating system.

Now, Microsoft has thrown Hyper-V into the mix too. Prime among the new features is live migration, a feature of VMware that has long been a thorn in the flesh for Microsoft. The new hypervisor offers live migration across different processors, a feature that was first revealed within Windows Server 2008 R2

According to Vijay Tewari's, principal program manager for Microsoft's virtualisation team, Hyper-V is the produc to cclose consultation with Microsoft customer. In his blog, he said that Hyper-V R2 offered users a range of additional features. T hese include: 64 logical processor support; the ability to support up 384 virtual machines; the ability to hot add or remove virtual storage and flash boot-up.