Page views and Impressions are used to measure two different items in the web world.
Page views are the count of number of times a page is viewed. The page can be viewed either directly by hitting the URL or by clicking on some other link through a search engine/another website.
Impressions are the count of the number of times an advertisement or banner which is placed on a webpage is displayed.
If we take an example, if a particular webpage having 2 banner ads has been viewed 1000 times, then the page view is 1000 but the collective impressions are 2000.