hibernate3-maven-plugin generates java.lang.ClassNotFoundException: org.hibernate.util.ReflectHelper

I am trying to generate a schema (ddl) from my orm.xml file. I'm using hibernate3-maven-plugin. I'm using JPA with hibernate as the implementation, but am open to other implementations or maven plugins. The code in my pom.xml is:

 <build>
    <plugins>
        <plugin>                
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>hibernate3-maven-plugin</artifactId>
            <version>3.0</version>

            <configuration>
                <hibernatetool>
                    <jpaconfiguration persistenceunit="unitname" />

                    <hbm2ddl export="false" create="true"
                 update="true" format="true" outputfilename="schemaDiff.ddl" />

                </hibernatetool>
            </configuration>
        </plugin>            
    </plugins>
</build>

The command I run is:

mvn hibernate3:hbm2ddl -e

The error I received is:

Caused by: java.lang.NoClassDefFoundError: org/hibernate/util/ReflectHelper at org.hibernate.tool.ant.JPAConfigurationTask.createConfiguration(JPAConfigurationTask.java:32) at org.hibernate.tool.ant.ConfigurationTask.getConfiguration(ConfigurationTask.java:54) at org.hibernate.tool.ant.HibernateToolTask.getConfiguration(HibernateToolTask.java:302) at org.hibernate.tool.ant.Hbm2DDLExporterTask.createExporter(Hbm2DDLExporterTask.java:51) at org.hibernate.tool.ant.ExporterTask.execute(ExporterTask.java:39) at org.hibernate.tool.ant.HibernateToolTask.execute(HibernateToolTask.java:186) at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:291) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106) ... 28 more

Caused by: java.lang.ClassNotFoundException: org.hibernate.util.ReflectHelper at org.codehaus.plexus.classworlds.strategy.SelfFirstStrategy.loadClass(SelfFirstStrategy.java:50) at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass(ClassRealm.java:244) at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass(ClassRealm.java:230) ... 40 more