This project shows how to develop a plugin that shows Google AdMob ads for Windows Phone and Xamarin.Android from a Xamarin.Forms project.
Supported projects: Xamarin.Android and Windows Phone 8 (Silverlight).
The resulted nuget package is available on: https://www.nuget.org/packages/Xam.FormsPlugin.AdMobView/
Here is a video on how to install the plugin for your Xamarin Forms project:
https://www.youtube.com/watch?v=RE4IbyLJtX0
Don't forget to add the following to your Android.Manifest inside :
< activity android:name="com.google.android.gms.ads.AdActivity" android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize" />