I'm on refactoring an existing app for Odoo 8. Even after reading some docs and previous posts I don't really understand how the model for configuration works works. I mean it works but I can't figure out how?

A see that there are two different ways to use res_config for a configuration model for an new app:

The old style?

class xyz_config_setting(osv.osv_memory):
    """ Setting days before to email alert """
    
    _name = 'xyz.config.setting'
    _inherit = 'res.config.settings'

New style?

 import logging
 from openerp.osv import orm 

 class ExchangeConfigSettings(orm.Model):

    _name = 'exchange.config.settings'
    _description = 'Exchange configuration'

    _inherit = 'res.config.settings'

1. What is in the second example appears I don't understand:  from openerp.osv import orm. Is this now ORM or OSV?

2. What about res_config in Odoo V9?

3. How ORM relates to new API. Is the API a part of ORM or can considered as equivalent? 

4. What are the benefits of using the class res_config and not just the new API (models.Model)? 
   

Would be glad to understand a bit better.