Sql Server Enterprise 2008 R2 Partition Switch with Xml Index
This is straight from BOL:  Transferring Data Efficiently by Using Partition Switching

XML columns must have the same schema. Any xml columns must by typed to the same XML schema collection.
No XML indexes allowed on the target table. There can be no XML indexes on the target table.

It appears that you need to ensure both XML columns are typed against the same schema and drop the XML index from the target partition, if applicable.
I have never personally performed a partition switch when an XML index is involved, so there could be more to it than this.
Hope this helps,
Matt