This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I think your relationship is actually many to one, not one to one i.e. many users will belong to one department.
So, in your userxml.hbm.xml file, change the relation to:
As a side issue, consider dropping the XML suffix from your class names. It doesn't follow standards and obscures their meaning. For example, your UserXML represents a User, your DepartmentXML a department.