Only two Xen Orchestra versions a maintained Stable and Latest.
They are both available directly from the web interface Using the release channel settings.
This is due to our development process where we can't maintain more than the current versions.
Stable 
The stable channel is intended to be a version of Xen Orchestra that is already one month old (and therefore will benefit from one month of community feedback and various fixes). This way, users more concerned with the stability of their appliance will have the option to stay on a slightly older (and tested) version of XO (still supported by our pro support).
Latest 
The latest channel will include all the latest improvements available in Xen Orchestra. The version available in latest has already been QA'd by our team, but issues may still occur once deployed in vastly varying environments, such as our user base has.