I think the simplest way to use JPA in OSGi is using Spring helpers.
Now you can inject jpaTemplate into your POJO services. If you don't want to use JpaTemplate then use entityManagerFactory.
If you want use JPA without Spring helpers then you should check Apache Aries JPA module (http://aries.apache.org/modules/jpaproject.html). Here is nice tutorial how to use "pure" JPA in OSGI: http://jaxenter.com/tutorial-using-jpa-in-an-osgi-environment-36661.html