너무 means "too much" It is more commonly used with a bad situation. But, nowadays, it is used with a good situation also colloquially. 매우 and 아주 are quite similar in the meaning. But the nuance is different. 아주 has stronger personal feeling than 매우 if 매우 means "very", 아주 would mean "very very". 매우 commonly used in written language with no emotion. 아주 commonly used in colloquial language with a personal emotion.