They are close, but not quite the same. Both a street vendor and street hawker sell items on the street. However, a street hawker is considered more aggressive. They will shout out what they have for sale and try to grab people's attention. In answer to your question then, a street hawker is a street vendor who makes more noise about what they are selling.

I "googled" the information. From what I read, both are the same thing there in Mongolia.