The xml export plugin for Coq has been discontinued for lack of users:
it was most certainly broken while imposing a non-negligible cost on
Coq development. Its purpose was to give export Coq's kernel objects
in xml form for treatment by external tools.

If you are looking for such a tool, you may want to look at commit
7cfe0a70eda671ada6a46cd779ef9308f7e0fdb9 responsible for the deletion
of this plugin (for instance, git checkout
7cfe0a70eda671ada6a46cd779ef9308f7e0fdb9^ including the "^", will lead
you to the last commit before the xml plugin was deleted).

Bear in mind, however, that the plugin was not working properly at the
time. You may want instead to write to the original author of the
plugin, Claudio Sacerdoti-Coen at sacerdot@cs.unibo.it. He has a
stable version of the plugin for an old version of Coq.