How is Citrix's upcoming desktop hypervisor different from its XenDesktop already on the table? Mainly, end users gain the ability to leave the network that connects their client to a host server and still operate their personal virtualized desktop. XenDesktop, on the other hand, is tethered, not free roaming.In September, VMware announced a similar initiative to Citrix with VMware vClient. It too puts a hypervisor on the end user's machine and ships an ESX Server-compatible version of the desktop down to it. It's also coming in the second half of 2009.

I can't help but think Citrix and VMware both saw the merit of several startups that have been experimenting with free-roaming desktops, able to run on different devices, such as RingCube and InstallFree. Both allow you to carry your essential desktop specifications around on a memory device in your pocket. Then you plug the device into a Windows machine somewhere and generate a working desktop, running in an insulated container. When you shut down, the desktop specs and data you were working with go back on the memory device rather than staying behind to be snooped on the guest machine. In contrast, Citrix XenDesktop and VMware's Virtual Desktop Infrastructure generated the desktop on a data center server, ran it there, and shipped responses from the central server down to the end user. RingCube and InstallFree cut the virtual desktop loose and gave it mobility. Control was secondary to roaming.

Both VMware and Citrix are seeking the best of both worlds as a way of unlocking the barriers to virtualizing millions of end users and cutting the cost of maintaining them. Mobility is no good without the ability to impose centralized policies and controls. It's already been amply illustrated how IT's lack of control over user laptops can lead to both mobility and stupendous corporate data loss. There's a tension between free roaming and control; too much of a one-sided resolution brings on new problems.

On the other hand, centralized control over end users without mobility isn't much good either.

Exactly how well VMware and Citrix each resolve the tension may well determine who gets to lead the next round of virtualization. VMware's approach is to supply a VMware View server to provision and manage end-user desktops. Under View, an upcoming, end-user hypervisor will check in with a command center to get updates and synchronize data upon reconnecting.

The details are still sketchy but Citrix will employ a similar concept, making its desktop hypervisor subservient to and reporting into a XenServer supervisor. The future isn't here yet, but the mobile and managed virtualized desktop is beginning to take shape on the horizon.