Why don't you make your xml field the Java type java.lang.Object.
This translates into the JDBC type JAVA_OBJECT and the SQL type BLOB via the standardjbosscmp-jdbc.xml configuration file in jboss/server/default/conf.
This worked for me.
Hope it solves your problem
This will not work in JBoss 3.0.4 onward.
Do not use JAVA_OBJECT unless you know exactly what it is for so that you can provide the support code for it.
See the JDBC doco for more information.
Change JAVA_OBJECT to VARBINARY or LONGVARBINARY if you have used it already. This is fully compatible with previous behaviour, as JBoss was effectively doing this under the covers in releases prior to 3.0.4 anyway.