Introversion is said to mean that a person gains there energy from solitude as opposed to extroverts gaining energy from being amongst other people.
Where as shy is defined as feeling nervous or timid around other people... this definition sounds alot like social anxiety.
Most of my life I have been introverted but growing up I was also very loud and outgoing in a crowd... in fact I quite enjoyed being social but also needed alot of alone time.
Shyness for me kicked in in my mid teens because I became too self concious. My worth was based on how outrageous others had percieved me in the past so as I my personality started to calm down I was really scared that people would no longer like me...
In experienceing both sides of the scale by living in a fearless state at first and then living in a fearful state I am very inclined to think that shyness is more often than not, a fearful state.
Introverted people who I have met and spoken to, once they understand their nature tend to feel quite accepting of that it in a social context. Where as people who I have spoken to share their experiences of being of feeling shy tend to talk about fearing how others may percieve them.