osv is a class and an OpenERP descriptor class and all the class( model) must inherit it for OpenERP module deployment.

osv class inside in OSV module in OpenERP server , which content all the OpenERP properties like you can see _column, _defaults and other many things there such as nameetc so we must inherit in our openerp model (class)

In version 7, 

The ORM, short for Object-Relational Mapping, is a central part of OpenERP.

In OpenERP, the data model is described and manipulated through Python classes and objects. It is the ORM job to bridge the gap -- as transparently as possible for the developer -- between Python and the underlying relational database (PostgreSQL), which will provide the persistence we need for our objects.
 

osv.osv is deprecated and it still works for backward compatibility. You should use orm.Model instead.

In version 8+, 
 

The model defnition is

orm.Model ---> models.Model

orm.TransientModel ---> models.TransientModel