Normal way In custom module create sepeate purchase.order.line form view, tree view and act_window action. In the menu define act_window action.  Menu Purchase Order Line record  view purpose set  create="false" in both form and tree view tag.

One way is to create the required menu from the Settings menu.

First, in order to display the model (purchase.order.line) you will first need to have a Window Action for that model.  In standard Odoo 7.0 (check from Settings >> Technical >> Actions >> Window Actions, filtering for Object purchase.order.line) you can see that the Purchases Action will display purchase.order.line (Destination Model) without any Domain.

Next, you'll need a Menu Item to access that Window Action.  From Settings >> Technical >> User Interface >> Menu Items, create a new Menu Item.  Give it a name, attach it to a parent menu (the menu under which this menu will be shown), and most importantly, in the Action field select ir.actions.act_window in the drop down box and select Purchases in the drop down box that appears next to the first drop down box.  After you save this menu, the new Menu Item will appear where it should be.  If you cannot find that menu, there might be some issues with access (Access Control List or access to the parent menus).

Note: view is not required.  If no view has been defined, Odoo will dynamically create the view when required.  If you need the information to be displayed in a specific way, you need to define the view.

If you define a new view, it is best if you also define a new Window Action so as not to affect the other displays.

The definition of view, actions, and menu item can also be included in an XML file in a module as Prakash had mentioned.