Hi,
As it seems your XML mapping file doesn't adhere to the XML schema as defined by the Bean Validation spec (see http://beanvalidation.org/1.0/spec/#xml-mapping-xsd). In particular you have to specify the <valid/> element before any <constraint> elements within <field> and <getter>.
To specify attribute values of constraints within an XML mapping, use the <element> element like this:
Code:
<field name="seatCount">
<constraint annotation="javax.validation.constraints.Size">
<element name="max">20</element>
</constraint>
</field>
You might also be interested in the chapter on XML configuration of the Hibernate reference guide (http://docs.jboss.org/hibernate/stable/validator/reference/en-US/html/validator-xmlconfiguration.html).
Hth,
--Gunnar