I have an app using Xamarin.Forms targeting IOS, Android and WP 8.
I need push notification feature in my app.
I have seen the pushsharp demos
pushsharp
You might look at the Appboy Component which has support for this out of the box. https://components.xamarin.com/view/appboy-sdk-bindings
As others have said, you can't do generically without some platform-specific components.