xen-source in the repositories seems to be pointing to the xen-source-2.6.16 package. Further, the xen-source-2.6.19 package (recommended by xen-image-2.6.19) is not found by aptitude.
installing the ubuntu-xen-server package installs the xen kernel based on the 2.6.19 ubuntu server kernel.
The following is my sources.list(without the comments)
deb http://us.archive.ubuntu.com/ubuntu/ feisty main restricted
deb-src http://us.archive.ubuntu.com/ubuntu/ feisty main restricted
deb http://us.archive.ubuntu.com/ubuntu/ feisty main restricted
deb-src http://us.archive.ubuntu.com/ubuntu/ feisty main restricted
deb http://us.archive.ubuntu.com/ubuntu/ feisty-updates main restricted
deb-src http://us.archive.ubuntu.com/ubuntu/ feisty-updates main restricted
deb http://us.archive.ubuntu.com/ubuntu/ feisty universe
deb-src http://us.archive.ubuntu.com/ubuntu/ feisty universe
deb http://us.archive.ubuntu.com/ubuntu/ feisty multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ feisty multiverse
deb http://us.archive.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu feisty-security main restricted
deb-src http://security.ubuntu.com/ubuntu feisty-security main restricted
deb http://security.ubuntu.com/ubuntu feisty-security universe
deb-src http://security.ubuntu.com/ubuntu feisty-security universe
deb http://security.ubuntu.com/ubuntu feisty-security multiverse
deb-src http://security.ubuntu.com/ubuntu feisty-security multiverse
deb http://medibuntu.sos-sts.com/repo/ feisty free non-free
deb-src http://medibuntu.sos-sts.com/repo/ feisty free non-free
deb http://archive.canonical.com/ubuntu feisty-commercial main
deb http://edevelop.org/pkg-e/ubuntu feisty e17
deb http://e17.dunnewind.net/ubuntu feisty e17
deb-src http://edevelop.org/pkg-e/ubuntu feisty e17
deb http://gnomefreak.youmortals.com/mozilla-testing feisty main
deb-src http://gnomefreak.youmortals.com/mozilla-testing feisty main
deb http://www.telemail.fi/mlind/ubuntu feisty main
deb http://kubuntu.org/packages/kde4-3.90.1/ feisty main
This can be closed now. linux-image-xen exists in gutsy.