Citrix Systems' Kensho Project has released an early version of its toolkit that can be used to produce virtual machine appliances that run under VMware's ESX Server, Microsoft's Hyper-V, or the Citrix XenSource unit's XenServer.

Kensho tools implement the Open Virtual Machine Format, or OVF, a specification that creates a neutral format in which virtual appliances -- or sets of virtualized files that represent an application with its operating system -- can be distributed. The purpose of OVF, according to the Distributed Management Task Force standards body, is to create a vendor-neutral "portability" format in which virtual machines may be moved between systems and run under different hypervisors.

The OVF portability package can tell a host's hypervisor what sort of files it contains and whether to construct a VHD-formatted virtual machine from the package to run under either Hyper-V or XenServer or a VMDK virtual machine to run under ESX Server. For that matter, the package may contain directions to construct several different virtual machines that represent a complete enterprise service or Web service, parts of which run on different systems.

"OVF is an export format for moving between VHD and VMDK file formats. They constitute 99% of the virtual machines out there," said Simon Crosby, CTO of the XenSource unit of Citrix. He was an early contributor to the OVF portability model. Citrix, Dell, Hewlett-Packard, IBM, Microsoft, and VMware all sponsored the draft of the OVF specification when it was submitted to the DMTF a year ago.

The toolkit is available in "technology preview" form as a free download. It works with VHD and VMDK files in its initial release, but there's no restriction on additional virtual file formats being added, Crosby said in an interview. The goal is for Kensho to become an open source project with its own community and core of developers, independent from Citrix, he added.

By using the toolkit, enterprise IT developers can produce an application, package it with an operating system in the OVF format, and then run it under the hypervisor of choice in different locations. Independent software vendors would use Kensho to package their code as a virtual appliance that could be run under any of the major hypervisors.

"OVF is a packaging format for an entire application stack," and the stack might include a database system virtualized on one system and application logic virtualized for another, he said.