This will be a quick one. After a fresh install of xenserver, you dont have any place to put your iso images (usualy they are on the NFS ISO storage, but you have to create it first). If there is a chance, you can set up a new SR for it and upload all you want to install, but there is a quicker way to install a debian like system.
XenServer has a set of templates, that you can use, and they determinate OS version which you want to install. The simplest and quickest way is to do it via http.
When creating new virtual machine, select template from template list. Let's say we want to install "Ubuntu Precise Pangolin 12.04 LTS".
After that set the name of your new vm and on the next screen you can pick the way your OS will be installed.
For ubuntu use : http://archive.ubuntu.com/ubuntu
For debian use : http://ftp.debian.org/debian