Erro ao gravar

Estou fazendo um projeto em jsf usando richfaces com o banco de dados postgre 8.3. Quando vou gravar ocorre o seguinte erro:

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javax.servlet.ServletException: #{usuarioBean.gravar}: javax.persistence.PersistenceException: org.hibernate.HibernateException: JDBC Driver class not found: org.postgresql.Driver

javax.faces.webapp.FacesServlet.service(FacesServlet.java:277)

org.ajax4jsf.webapp.BaseXMLFilter.doXmlFilter(BaseXMLFilter.java:154)

org.ajax4jsf.webapp.BaseFilter.handleRequest(BaseFilter.java:260)

org.ajax4jsf.webapp.BaseFilter.processUploadsAndHandleRequest(BaseFilter.java:366)

org.ajax4jsf.webapp.BaseFilter.doFilter(BaseFilter.java:493)

org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)

javax.faces.FacesException: #{usuarioBean.gravar}: javax.persistence.PersistenceException: org.hibernate.HibernateException: JDBC Driver class not found: org.postgresql.Driver

com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:118)

javax.faces.component.UICommand.broadcast(UICommand.java:394)

org.ajax4jsf.component.AjaxViewRoot.processEvents(AjaxViewRoot.java:317)

org.ajax4jsf.component.AjaxViewRoot.broadcastEvents(AjaxViewRoot.java:292)

org.ajax4jsf.component.AjaxViewRoot.processPhase(AjaxViewRoot.java:249)

org.ajax4jsf.component.AjaxViewRoot.processApplication(AjaxViewRoot.java:462)

com.sun.faces.lifecycle.InvokeApplicationPhase.execute(InvokeApplicationPhase.java:82)

com.sun.faces.lifecycle.Phase.doPhase(Phase.java:100)

com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)

javax.faces.webapp.FacesServlet.service(FacesServlet.java:265)

org.ajax4jsf.webapp.BaseXMLFilter.doXmlFilter(BaseXMLFilter.java:154)

org.ajax4jsf.webapp.BaseFilter.handleRequest(BaseFilter.java:260)

org.ajax4jsf.webapp.BaseFilter.processUploadsAndHandleRequest(BaseFilter.java:366)

org.ajax4jsf.webapp.BaseFilter.doFilter(BaseFilter.java:493)

org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)

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<?xml version="1.0" encoding="UTF-8"?>  
   <!-- example of a default persistence.xml -->
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"
	version="1.0">
	<persistence-unit name="cooperativa">
		<properties>
			<property name="hibernate.hbm2ddl.auto" value="none" />
			<property name="hibernate.format_sql" value="true" />
			<property name="hibernate.dialect" value="org.hibernate.dialect.PostgreSQLDialect" />
			<property name="hibernate.connection.driver_class" value="org.postgresql.Driver" />
			<property name="hibernate.connection.username" value="xxxxx" />
			<property name="hibernate.connection.password" value="xxxx" />
			<property name="hibernate.connection.url" value="jdbc:postgresql://localhost:5432/cooperativa" />
		</properties>
	</persistence-unit>
</persistence>

package br.com.cooperativa.beans;

import br.com.cooperativa.persistence.BaseDAO;
import br.com.cooperativa.util.ServletUtil;
import br.com.cooperativa.vo.Usuario;

public class UsuarioBean {

	private Usuario usuario = new Usuario();

	public void setUsuario(Usuario usuario) {
		this.usuario = usuario;
	}

	public Usuario getUsuario() {
		return usuario;
	}
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	public void gravar(){		
		BaseDAO baseDao = new BaseDAO<Usuario>();
		baseDao.create(this.usuario);
	}
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}

<ui:composition template="/templates/internal.xhtml">
			<ui:define name="body">
			<h:form id="idFormInclusaoUsuario">
				<table align="left" width="100%" border="0" cellspacing="1"	cellpadding="1">				
				<tr>
					<td>
						<h:outputLabel value="Nome:" style="font-size: 14px;"/>
						<h:inputText required="true"
						value="#{usuarioBean.usuario.nome}"
						size="20" maxlength="20" label="CR" id="nomeUsuario" />
					</td>
				</tr>
				<tr>
					<td>
						<h:outputLabel value="SobreNome:" style="font-size: 14px;"/>
						<h:inputText required="true"
						value="#{usuarioBean.usuario.sobreNome}"
						size="20" maxlength="20" label="CR" id="sobreNomeUsuario" />
					</td>					
				</tr>
				<tr>
					<td colspan="4" align="center">
						<h:commandButton value="Enviar"	action="#{usuarioBean.gravar}" />
					</td>
				</tr>
				</table>
			</h:form>								
			</ui:define>
	</ui:composition>