A lot has happened in the last several months of 2009 since last we last compared virtual servers. Citrix has made Xen Server free and released a new version 5.5 in mid 2009. Microsoft has released R2 of Windows Server 2008 that extends Hyper-V in new and useful ways, and adds support for Red Hat Linux guest OSs finally. And VMware has come out with about 57 new products, mostly labeled as version 4, to manage and integrate their line, and handle greater densities of VMs per server.
Before we rate the various virtualization vendors, here are some issues that are either no longer as relevant or that haven't much changed from last year's virtualization comparison:
Part of the problem with the pre-installs was a confusing story from VMware, and a lack of OEM focus from Citrix for Xen.
Most OEMs want to build their own servers anyway, so they can get more money from you on services and support on post-sale anyway. Look instead in 2010 for a few vendors to start shipping specialized virtual servers, like the Cisco Unified Communications Platform that begun shipping in mid-2009. These will be able to densely pack VMs by the truckload perhaps as much as terabyte RAM densities, which are needed to support dozens or hundreds of VM instances.
Microsoft's Hyper-V Server 2008 R2 can handle new CPU features called Rapid Virtualization Indexing by AMD and Extended Page Tables by Intel that will boost performance for VMs running on those machines.
VMware is still in the best position here, although Xen at least puts up this page that shows you how to assemble a pre-built appliance.
Citrix is focused more on its Project Kensho, which enables import and exporting virtual appliances in the Open Virtual Machine Format that it shares with Microsoft. Citrix also has a series of Amazon Elastic Cloud resources called the Citrix Cloud Center that puts all of its various network and virtualization tools in the cloud for potential customers to experiment with.
VMware supports OVF with its vApp utility, so look for better interoperability if you want to switch hypervisors or manage mixed environments.