A pop icon is a music star who has left an indelible stamp or personalized mark on music such that their music identifies a whole new dimension. Pop icon's music styles are widely imitated; however, everyone knows who the true originator is -- the pop icon himself. Think of the king of pop, the late Michael Jackson, as a pop icon.
A pop star is also a music star; however, their notoriety is oftentimes short-lived. Even though they are recognized because of their specific sound, the sound is not quite unique. Also their music has not left any lasting impressions in the music business -- except maybe during the time when their music was highly popular. Think Britney Spears as an example.
A pop idol is unfortunately at the low end of the pop music hierarchy. You listed the pop sequences correctly, by having the pop idol at the bottom. A pop idol's mark in the music field has an even lesser time-span than the pop star. Because of the wide-spread popularity of American Idol, it's fitting to tie in some of the idols who have won in the past -- some who are no longer in the limelight. The exception is of course, Kelly Clarkson, the first American Idol winner who continues to churn out hit records. Also with her stunning performance, in my opinion, of the national anthem during the super bowl, she may be on her way to becoming a pop star. That being said, for pop idol -- think Kelly Clarkson.