when you generate your hbm file ,your auto  generated primary  key is like this:

<id name="id" type="long">
       <column name="ID" />
       <generator class="native" />
</id>

In Hibernate mapping file

using this code  we can automatically generate the primary  keys

<id name="emp number" type="integer">
<column name="emp number"/>
<generator class="increment"/>
</id>

There are some algorithms that can be used for PK generation, please have a look at:
www.hibernate.org/.../mapping.html

Some of them can be used on specific databases, while others like 'increment' can be used for all databases. There are some performance considerations with this approach.