ORA-16000 when trying to perform select on read only Oracle database
I am not aware of any specific privileges related to XMLType. There will be execute privs related to PL/SQL that works with it but your message is clear and sadly definitive. If you had a privs issue you wouldn't be getting this message...

XMLType can be funny to work with. Have you thought about retrieving it as a CLOB and then doing the XML parsing and extraction at the client end?