Exactly, your inheritance strategy is not single table, or "Table per class heirarchy" which is what the discriminator value is used for.
here are the three inheritance strategies.
http://docs.jboss.org/hibernate/stable/core/reference/en/html_single/#inheritance-strategies
If the data model is try third normal form, then you want to use the table per concrete class strategy.
If you only have the three tables, one for each Subclass type of review then you want to use the table per subclass strategy. And the only real difference is the tag that you use in the mapping <joined-subclass> or <union-subclass>
And the easy way to know which one is the type of query that would need to be run to get one set of all the records from all the tables.
So if there is no "parent" table for "review" then you have a table for each subclass, so the query in the backend that needs to run will have Union of queries. One query for each table all unioned together.
If it is third normal form, then the tables in the query will all be in the from of a single query with joins.
Hope that helps
Mark