Overy Timid Dog?

My dog is friendly, playful, and energetic with my son, my husband, and myself, but is so timid with visitors that she hides. She barks at door knock/door bell, but once guests are inside she'll run and hide. She's a Maltese-Yorkie-Poodle mix. She is just about 2, from a litter of 5 pups. When encountering other dogs, she barks at a distance, but when they come up to greet, she goes into timid mode again. What can I do to help her with this extreme timidness?

By Wanda