Tim Freeman from GridVM.org provides us with this piece of information.  According to them, Keir Fraser writes on xen-devel:

The imminent next Xen release introduces a host of important new features including PV 32-on-64, HVM save/restore, and XenAPI 1.0. Now is a good time to bump our version number and reclaim the redundant second digit! We plan to rename the xen-3.0.5-testing.hg tree to xen-3.1.0-testing later today. The release candidate will be renamed to 3.1.0-rc4. The final release will be called 3.1.0 (as opposed to 3.0.5-0 in the old numbering scheme). Further bug-fix releases in the 3.1 series will be called 3.1.x (as opposed to 3.0.5-x in the old numbering scheme). This change rationalises and simplifies our version numbering scheme. It avoids the need for the fourth digit “-x”, and incrementing the second rather than third digit makes it clearer that large changes, not just bug fixes, are incorporated in our quarterly major releases.

You can leave a comment, here.  Thanks Tim!