The cross platform app development with Xamarin Forms allows to create multiple platform development projects under a single Visual Studio solution. It is easy to build app for Android and Windows Phone in Windows with Xamarin using Visual Studio. But for the iOS development we need XCode, which can only be installed in a MAC environment due to its dependencies. Xamarin has provided a tool called build host to support iOS development from Visual Studio in Windows.
This video demonstrates how to connect the Xamarin build host from MAC with Visual Studio in order to develop an iOS app.