Consider the hierarchy :

And the following classes and xml :
Instead of using HibernateUtil.java, to create sessionfactory object, you should use this:
SessionFactory sessionFactory=new AnnotationConfiguration().configure().buildSessionFactory();
Because in order to avoid the exception, you'll have to declare the class object in HibernateUtil.java file as configuration.addAnnotatedClass(Student_Info.class); which looks dumb because we have provided the entry already in hibernate.cfg.xml file.
To use the AnnotationConfiguration class you'll have to add a jar to your project build path: http://www.java2s.com/Code/Jar/h/Downloadhibernate353jar.htm