Hibernate has many configuration properties. For the properties that you do not specify in the persistence.xml file, JBoss AS will provide a reasonable set of default values. The default Hibernate property values are specified in the JBOSS_DIST/server/default/deploy/ejb3.deployer/MEAT-INF/persistence.properties file. Below is the persistence.properties file bundled in JBoss AS 5.0. Notice the options that are commented out. They give you an idea of available properties in your persistence.xml file.

hibernate.transaction.manager_lookup_class=org.hibernate.transaction.JBossTransactionManagerLookup
#hibernate.connection.release_mode=after_statement
#hibernate.transaction.flush_before_completion=false
#hibernate.transaction.auto_close_session=false
#hibernate.query.factory_class=org.hibernate.hql.ast.ASTQueryTranslatorFactory
#hibernate.hbm2ddl.auto=create-drop
#hibernate.hbm2ddl.auto=create
hibernate.cache.provider_class=org.hibernate.cache.HashtableCacheProvider
# Clustered cache with TreeCache
#hibernate.cache.provider_class=org.jboss.ejb3.entity.TreeCacheProviderHook
#hibernate.treecache.mbean.object_name=jboss.cache:service=EJB3EntityTreeCache
#hibernate.dialect=org.hibernate.dialect.HSQLDialect
hibernate.jndi.java.naming.factory.initial=org.jnp.interfaces.NamingContextFactory
hibernate.jndi.java.naming.factory.url.pkgs=org.jboss.naming:org.jnp.interfaces
hibernate.bytecode.use_reflection_optimizer=false
# I don't think this is honored, but EJB3Deployer uses it
hibernate.bytecode.provider=javassist