You can uninstall or removes an installed kfreebsd-image-8.3-1-xen package itself from Debian 7 (Wheezy) through the terminal,
$ sudo apt-get remove kfreebsd-image-8.3-1-xen
If you would like to remove kfreebsd-image-8.3-1-xen and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove kfreebsd-image-8.3-1-xen
If you use with purge options to kfreebsd-image-8.3-1-xen package all the configuration and dependent packages will be removed.
$ sudo apt-get purge kfreebsd-image-8.3-1-xen
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove kfreebsd-image-8.3-1-xen