I can't seem to find any mention of the differences between the two?
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As far as I can tell, Citrix XenServer offers the following on top of Xen.org:
@ceejayoz's answer is correct on the technical points. Note, however, that both xen.org and XenServer are available for free (see http://www.citrix.com/freexenserver). The value-add features Citrix offers may or may not be required in your environment, but I've found from personal experience that the full XenServer works very well for small-scale deployments.
XenServer will support Windows up to and including Windows 2008. Official Windows 7/2008 R2 support should be introduced soon; it does currently work, albeit with a command-line tweak, and it's not officially supported.
There is no difference in terms of Xen. They both provide the same xen hypervisor. It's what you get in addition that is important.
Citrix have taken Xen and are attempting to make it as user friendly as ESX - that means making it straight forward to use by most admins without them needing to delve in to linux on dom0 to get everything working. It's the management tools that are key. Have a look at the cli options for 'xm' and Citrix 'xe'. You can do so much more with 'xe'. Also take a look at the XenServer API for management and monitoring
So you have the pretty gui - although this was nothing compared to the VI client last time I checked. More importantly most of the dirty work that you need to do in dom0 for a stock xen install is hidden in Xen Server. Networking is a good example, you can do bridging and vlan trunking by hand in dom0, however XenServer hides this. You also have 'clustering' of xen hosts with XenServer, you'll need to write your own otherwise.
XenServer makse xen an option for companies that aren't stuffed full of techies who are happy to roll their own.
This matrix is the best active summary of the differences: http://wiki.xensource.com/xenwiki/XCP/XenServer_Feature_Matrix
See also (a related question): Deploy Xen (In particular, see the part about Project Kronos)