Of the BSDs NetBSD has by far the most mature support for Xen, supporting 32-bit, 64-bit, and dom0. There is no indication that it supports PAE guests.
You can find out more about it on the NetBSD site.
The current maintainer is Manuel Bouyer.
The development of support for FreeBSD has been rather off and on. There is currently active development going on, but it remains to be seen if it will develop the same level of maturity as NetBSD. It currently *only* runs as a domU in PAE mode.
You can find out more about on the FreeBSD wiki.
The developer is Kip Macy.
An OpenBSD port was done. Unfortunately, Theo de Raadt does not feel that it is something he wants as part of the tree.
Cheers,
BSD Tech
