The XML view does not shows the Full Name. Steps to replicate:

1. create a User Picker field and add it to a screen
2. create a new user. Let's say username="jalbion" Full Name="Janet Albion"
3. create an issue and set choose "jalbion" in the User Picker type field
4. go to Issue Navigator
5. add User Picker type custom field to the column
6. search for the issue in the Issue Navigator
7. choose to view the Issue in the XML view and the XML will return username:

   <customfields>
   <customfield id="customfield_10100" key="com.atlassian.jira.plugin.system.customfieldtypes:userpicker">
   <customfieldname>userpicker</customfieldname>
   <customfieldvalues><customfieldvalue>jalbion</customfieldvalue></customfieldvalues>
   </customfield>
   </customfields>