Start an Android activity in Xamarin Forms?

You can use DependencyService to implement this function:

INativePages in PCL:

 public interface INativePages
{
    void StartActivityInAndroid();
}

Implement the interface in Xamarin.Android:

[assembly: Xamarin.Forms.Dependency(typeof(NativePages))]
namespace PivotView.Droid
{
    public class NativePages : INativePages
    {
        public NativePages()
        {
        }

        public void StartAc()
        {
            var intent = new Intent(Forms.Context, typeof(YourActivity));
            Forms.Context.StartActivity(intent);
        }

    }
}

Start an Android Activity in PCL :

    private void Button_Clicked(object sender, EventArgs e)
    {
        Xamarin.Forms.DependencyService.Register<INativePages>();
        DependencyService.Get<INativePages>().StartAc();
    }