Paid Previews Vs Premier

1. Viewing before actual release is known as preview if paid money for it. 
2. Viewing arranged by producer or others related to film for close ones then it is called premier.

https://www.youtube.com/results?q=paid+previews

Premiers are organized normally for free where passes are distributed for very few selected audience in order to get an early response . In paid previews, the film is made available for general audience, instead of friday the film opens on thursday itself on selected theaters only where the makers think that more business can be made.

https://www.youtube.com/results?q=Movie+premiere