I think it has been widely shown that Muslims do not use the word infidel, and that the closest Arabic equivalent is Kafr. Kafr is a word used to describe not those who disbelieve in Islam, but rather those who realize in their minds and/or hearts that Islam is the truth but reject it anyways. That is the only true definition of the word Kafr. In fact, that's where we get the English word "cover" -from Kafr, because Kafr's "cover" the truth. Regarding other types of disbelievers, there are varying opinions on this matter. The opinion I follow is that of Imam Ghazali who placed 2 out of 3 categories of disbeliever in paradise, and placed only the Kafr category of disbeliever in the hellfire.