Cocos2D for XNA is a C# Port of the Cocos2D Framework using MonoGame behind the Scenes.
Cocos2D for XNA is a C# Port of the Cocos2D Framework. XNA got discontinued by Microsoft, and doesn't get supported in newer VS Versions (2010 Last). Cocos2D for XNA now runs on the MonoGame platform, that is an OpenSource successor of XNA, that runs on many platforms, so don't get mislead by the name of the Framework.