This example would run the hbm2ddl goal whenever the process-classes phase gets invoked.
<project>
...
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>hibernate3-maven-plugin</artifactId>
<version>2.1</version>
<executions>
<execution>
<phase>process-classes</phase>
<goals>
<goal>hbm2ddl</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
...
</project>