Linux' two main distributors, Novell and Red Hat, have embarked on a war or words, trading insults over one another's virtualisation strategies.

At the centre of it is the open source Xen virtualisation software, designed to run multiple servers on a single hardware platform.

In recent months, Xen has begun to be seen as a potential alternative to VMware, which dominates x86 server virtualisation - partly because of the enthusiastic backing Xen received from giants such as Red Hat, Novell, Sun and IBM.

Last October, Red Hat even announced it would push for Xen to be integrated into the Linux kernel and in March, took the wraps off its ambitious "Integrated Virtualization" plan, all based on Xen.

Recently, however, just as Xen-based products actually began to arrive on the market, Red Hat had an apparent change of heart, with executives saying Xen isn't suitable for enterprises' mission-critical systems.

As a result, Red Hat's first Xen-based product won't arrive until late this year or early next year, when it ships Red Hat Enterprise Linux (RHEL) 5 with Xen integrated.

Novell shipped Xen in SuSe Linux Enterprise Server (SLES) 10 last month, a decision Red Hat chief technology officer Brian Stevens called "cavalier" in a report published last week in the industry journal eWeek.

He said: "I think they [Novell] are being cavalier. We know what we need to be enterprise-ready and we already have a checklist of everything we need for that. They have decided it's more important to be first. We should not screw this thing up and put a cloud around Xen."

Novell chief technology officer Jeff Jaffe responded that Red Hat's remarks have more to do with its own late products than with Xen. "Red Hat has adopted the John Kerry approach to virtualisation: constant flip-flopping," he said in a report from Linux Watch.

Insurance giant Nationwise is already using SLES and Xen in production IBM zSeries mainframes, according to Novell.

Virtual Iron, which is making standalone virtualisation products based on the Xen engine, has also blasted Red Hat. The company pointed out that Xen itself is just a project, which needs finishing and integration work from distributors to make it usable. Novell claims to have put in that work with SLES 10.

XenSource, the commercial company which leads Xen development, was more conciliatory. The company admits it will take a while for Xen to make its way into the data centre, but said it is perfectly fine for test and development work - the area where VMware got its start.

XenSource is to ship its first Xen-based product, XenEnterprise, this week. XenEnterprise is designed to allow multiple operating system instances to run on "bare metal" servers - that is, without an underlying "host" operating system.

When paired with hardware virtualisation support from Intel or AMD the system should support Windows as well as Unix-like operating systems.

In a sign that the initial shine is wearing off of Xen, others have begun picking holes in the project. One analyst, Gordon Haff of Illuminata, highlighted problems with aspects such as security and the performance profiler - although his comments were prompted by information from Red Hat.

Haff's overall take was that "Xen's current state need not reflect its future prospects. New technologies take time to mature. But headlines and press releases about Xen having arrived notwithstanding, make no mistake about it. This is an experimental product. Kick the tyres if you must but stay off the highway."

At one point, it was looking as if Xen would migrate into the Linux kernel and become a standard part of the system. IBM has already committed to it.

However, questions were raised following XenSource's recent agreement with Microsoft. The agreement allowed a para-virtualised Windows to inter-operate with Xen, and significantly add to Xen's appeal by making Windows Server virtualisation able to run open source Xen-based guest operating systems.

The deal was promptly slated by virtualisation market leader VMware, which called it a "one-way street," and accused XenSource of betraying its open source roots. VMware said XenSource would be optimising Microsoft code to run on the Xen hypervisor while not allowing Microsoft-XenSource developed code to be used by the open source community.

Brian Byun, VMware's VP of products and alliances, pointed out in his blog that the deal is "a one-way street that favours Microsoft and Windows running Linux. The arrangement will allow Linux to run on future Microsoft hypervisors through translated calls to the hypervisor when Windows is controlling the hardware, but not the other way around.

That means there is no mention of Longhorn optimisations or 'enlightenments' being ported to Xen or licensed to XenSource to enable a Xen hypervisor to run full optimisations with Longhorn OS."

So XenSource's Microsoft deal plus its alleged technological failings might result in it being discounted as the virtualisation system of choice for the Linux kernel. Instead, SWsoft, which makes Virtuozzo, could well supplant it.

Techworld's Manek Dubash contributed to this report.