Much of the reason, analysts say, is the increasingly common refusal among customers to stick to just one vendor's virtualization approach, analysts say.

"Several of our clients are looking at using Microsoft's Hyper-V or Citrix XenServer to compliment their existing VMware ESX environment," says Chris Wolf, senior analyst for The Burton Group in Salt Lake City. â¬SThe reasoning has primarily been cost, and the other platforms provide the services they need for virtualizing branch offices as well as development, test or training environments."

Citrix Systems and Microsoft's management tools—XenCenter and System Center Virtual Machine Manager 2008—both manage not only their own XenServer and Hyper-V virtualization products, but virtualized environments that run using VMware's ESX Server.

Citrix's XenCenter manages the company's XenServer, VMware's ESX Server and Microsoft's HyperV environments. Microsoft has vowed with its upcoming Virtual Machine Manager 2008, which is in beta test now, to also support Xen and VMware ESX Server implementations.

"It's not just about managing VMware enough to get customers to convert to Hyper-V, we are looking at a full management experience for both sets of users," says Ed Yuen, technical product manager at Microsoft.

"We've drawn the route of having multi-vendor management," says Yuen. "We recognize that a customer isn't going to have just Microsoft Hyper-V servers or virtual servers, they are going to have VMware racks too—and if they do, VMM will manage it."

Yuen says that Microsoft made a commitment to provide Xen management. "Obviously since the acquisition of XenServer, Citrix would be at the top of that," says Yuen.

VMware is the only virtualization vendor in the top three who hasn't agreed to managing other environments, but even it hasn't ruled out the possibility.

"At this point, it is definitely something we want to do, when we feel there is enough market adoption of other virtualization technologies," says Erik Wrobel, director of product management for VMware. "Right now when we talk to customers they either have not adopted mixed environments at all or have done so in small amounts."

That hesitation is largely because VMware has little to gain by making it easier for customers to use software from its competitors, Wolf says.

"VMware currently doesn't have as much to gain by supporting products with small market penetration," says Wolf. "Rushing to support these platforms could also be viewed as VMware validating their relevance, which is something VMware is in no hurry to do."