Microsoft has moved to improve its woeful image in the open source community, striking a deal that will allow a para-virtualised Windows to interoperate with XenSource's Xen.

The deal could add significantly to Xen's appeal by making Windows Server virtualisation able to run open-source Xen-based guest operating systems - including various Linux distributions and, as of next year, Solaris 10.

Microsoft will also offer technical support for Windows-Xen interoperability issues. Microsoft has already agreed to offer "reasonable" technical support for instances of Windows running as guests on XenSource's Xen Enterprise.

The deal doesn't involve Microsoft's current Windows Virtual Server products, but relates to the "hypervisor" the company is developing for Windows Server "Longhorn", which is set for release at the end of next year.

Microsoft plans to release a beta of the hypervisor, code-named Viridian, by the end of 2006 and will release the final version 180 days after Longhorn's debut. Virtual Server 2005 R2 currently supports Linux guest operating systems.

Server virtualisation is generally aimed at running more than one instance of a server on a single physical machine. With Windows Server virtualisation and Xen, the x86 server industry is shifting towards para-virtualisation, which offers a very low processing overhead, but requires modification of the OS.

Xen is being built into the next releases of Red Hat Enterprise Linux, SuSe Linux Enterprise Server and other Linux distributions, and will be supported by Solaris 10 next year.

Xen Enterprise doesn't need an underlying server OS, running on bare metal hardware. The introduction of hardware-based virtualisation in newer AMD and Intel chips means Xen Enterprise and the Xen built into Linux distributions can both run Windows as a guest OS.

Currently, x86 virtualisation is dominated by VMware, which has more or less invented the market since 1998. According to XenSource, formed by Xen's original developers, Microsoft decided the best way to catch up with VMware was to make its products freely available and interoperable.

"Microsoft has absolutely got the interoperability message," Simon Crosby, chief technology officer of XenSource, told Techworld. "When they finally woke up to virtualisation, they knew they needed to go flat out and work with the best companies out there."

Microsoft made its virtual hard disk (VHD) format freely licensable, and XenSource has taken Microsoft up on the offer, meaning its products can access images created by Virtual Server.

XenSource's Peter Levine said the company is planning more products based on technology collaboratively developed with Microsoft.

"Microsoft's commitment to customers is to build bridges across the industry with solutions that are interoperable by design," said Bob Muglia, senior vice president of the Server and Tools Business at Microsoft, in a statement.