It's generally not a good practice to call your business classes directly from javascript.
But you can place a mediator (business delegate) java class inbetween and can access the session bean instead.
To access any java class using ajax, you may need a remoting library like DWR or JSON-RPC.
These libraries offer very powerful and easy way to use ajax in your application.
But if you want a more simpler solution, then have a look at libraries like "ajax4jsf" or "ajaxanywhere".