Adding a new OS disk images to XVM installable as HVMs is quite simple:
simply download a trusted ISO of the os, create the image from it, and add the image to the database.

note: see http://linux.die.net/ for man pages you don't have installed locally.

There is in fact a package invirt-images for doing this, but
I'll walk through the process with Fedora 14 as an example for your edification.

{{{root@aperture-science:~# wget http://mirrors.mit.edu/fedora/linux/releases/14/Fedora/i386/iso/}}}


{{{root@aperture-science:~# du -sh Fedora-14-i386-DVD.iso}}}

In this case we find that the image is 3.5G, which we need to know to create the logical volume in the volume group named xenvg.

{{{root@aperture-science:~# lvcreate -n image_fedora-14-i686 -L 3.5G xenvg}}}                       

Having created the space in the volume, we now need to place the image in it, reading from the ISO.

{{{root@aperture-science:~# dd if=Fedora-14-i386-DVD.iso of=/dev/xenvg/image_fedora-14-i686 bs=4M}}}

and then just clean up the old image   
{{{root@aperture-science:~# rm Fedora-14-i386-DVD.iso}}}
                                         
In order to allow users to install a copy of Fedora 14 from the web interface, invirt-web, and consequently invirt-database need to know about it. 
We simply enter it into the database using the postgres shell on xvm.

{{{root@xvm:~# sudo -u postgres psql invirt}}} 
  

{{{invirt=# insert into cdroms values ('fedora-14-i686', 'Fedora 14 (i686)', 'fedora', '14/Fedora')}}}

{{{invirt=# \q}}}

{{{root@xvm:~# exit}}}



Adding new Autoinstalls is somewhat more complicated.
Someone (quentin?) should fill in what is required to do that.