Well we can theorize the difference between insecure shyness and secure shyness all we want. But to be honest, I have yet to meet someone who is Shy but Secure.... Obviously we get to the discussion between being introvert or shy. In my experience shyness is always the insecure side, people are afraid to open up, and say what they want.
Introverted people are just to themselves and can't be bothered. Often this introvercy becomes shyness when someone is insecure.
The difference between shyness and being introverted is whether or not someone is conscious in being quiet, calm and to themselves.
Another thing I noticed is that with shy people it tends to be very hard to communicate and have a conversation. How romantic it may seem that it's like a present you can unwrap, communication is still essential for any relationship (romantic and non-romantic).
Like everything in life, there should be balance. It's far from bad to be calm, and not a shoutout, but when it becomes hard to communicate normally and to the point with you, it becomes a really annoying treat.
But that's just my experience and opinion
