With the recent acquisition of XenSource by Citrix, it seems like a good idea to take a look at the history of Xen and XenSource so we have our bearings once the dust settles. This will be a brief look, but I'm sure more details will come out from you "Xen Experts" in the comments.
First off, XenSource is a commercial company that sells a suite of enhancements and management products that augment an open-source hypervisor called Xen. The lead developer for XenSource, Ian Pratt, is also the chief architect of the Xen open-source project - a competitive leg up by no small means. In fact, XenSource is the key corporation behind the Xen hypervisor, so any enhancements to Xen also enhance XenSource (and anyone else who uses Xen, for that matter).
XenSource's goal, to put it unofficially, is to provide a solution akin to VMware, but using an open-source hypervisor. This hypervisor has differences from VMware that some call advantages. Some also call them disadvantages, which has led to what appears to be an epic battle akin to NT/Novell circa 1996. More on this battle in future articles, I'm sure. XenSource originally started as a pay-for support option for organizations that wanted to run the Xen hypervisor. Over the last few years, they have created an impressive suite of management solutions for Xen, and have even collaborated with Microsoft.
Xen, first released to the public in 2003, uses a type of virtualization called "paravirtualization." This is essentially a software method of interfacing virtual machines to the host hardware...sort of an API. Because of this, Linux operating systems have to have modifications made to run as a guest on a Xen server (called "Xen-enabled"). This is the part that sparked that battle between the VMware guys and the Xen guys. VMware uses a method called Binary Translation, which uses hidden hardware instructions to accomplish the same thing (that is, functional virtual machines).
With the advent of Intel VT and AMD SVM, Xen now supports Windows virtual machines without changes to the guest OS (Windows virtual machines were not supported prior to support for VT and SVM). This has brought the virtualization methods of the two companies a little closer together, but still fundamentally different.
Note: I tried really hard to get that right, but if I missed something or am way off base, please email me and let me know the real deal.
From 2003 to 2005, Xen's relative youth, it was developed into a popular desktop hypervisor. Able to support only one 32-bit processor, there wasn't much enterprise appeal. This is when XenSource was simply a commercial Xen support company. In 2005, XenSource released Xen v3, the first enterprise-class release of Xen (even though it was called version 3, it was really the first). With this release Xen could run on servers with up to 32 processors, and was the first version with built-in support for Intel's VT technology. AMD hadn't released SVM yet, but it too was eventually supported. In addition to the processor enhancements, Xen v3 also introduced support for Physical Address Extensions (PAE) to support 32-bit host servers with more than 4GB of memory. At this point, Xen still only supported Xen-enabled Linux guest operating systems.
The v3 release of Xen also resulted in XenSource's first legitimate approach to an enterprise solution - XenOptimizer. XenOptimizer was intended to be bridge the gap between Xen and VMware. Remember, Xen is just the hypervisor, XenOptimizer from XenSource (confusing, eh?) was the management interface. With XenOptimizer, admins were now able to manage multiple servers from a common interface for server provisioning and resource control.
In late 2006, XenSource released its first version of XenEnterprise 3.0, a product meant to directly compete with VMware. Based on v3.03 of Xen, it included an new management and monito