How do a shy person and a person with autism differ?

Quite the opposite. A shy person has an intuitive understanding of social cues and social implications of language (may include subtle gestures and tone of voice). The shy person experiences unease in social situations, which may lead to behaviors of avoidance (e.g. Averting eye gaze). In contrast, a person with autism is not readily attuned to social and language cues. The social withdrawal is not intentional.
A lot. Social impairment/communication difficulties/repetitive and stereotyped behaviors are the core symptoms of autism. A shy person may not look you in the eyes, but will show none of the other core symptoms. The social impairment in a shy or anxious youngster is demonstrably much less than in an autistic child.