_imageView.Layer.CornerRadius = _imageView.Frame.Size.Height/2;
_imageView.Layer.MasksToBounds=true;
// if you want a border
_imageView.Layer.BorderColor=UIColor.Blue.CGColor;
_imageView.Layer.BorderWidth=1.1f;You will learn how to place a circle around the UIImageview in Xamarin.iOS.
