I got this error when I was adding new functionality to my "Teach myself NHibernate" application.
I binged, googled for several hours....
Some of popular suggestions include, setting the xml file as an "Embedded resource", checking for typos, making the classes public, but those were not my problems. Most of them solved their problem easily by fixing one of those things. But my problem wasn't that simple or so I thought.
I tried everything under the sun and finally it came to me that I named my file PRODUCT.XML instead of PRODUCT.HBM.XML.