What is the difference between "denizens" and "residents"?
Historically, in British English, a denizen referred to an immigrant (a resident alien) who resided on foreign soil ‘The denizen was not a citizen nor an alien: but had a status akin to permanent residency today.’
Late Middle English deynseyn, via Anglo-Norman French from Old French deinz ‘within’ (from Latin de ‘from’ + intus ‘within’) + -ein (from Latin -aneus ‘-aneous’). The change in the form of the word was due to association with citizen.
(English Oxford Dictionaries)
A resident is usually someone who lives at a fixed abode or permanent address. You can have a residence in numerous locations but you can only be called a resident if you spend considerable time in one of these homes.
“He entered the United States in 1988 as a permanent resident because of his marriage to a U.S. citizen.” (CD)
Delving into Wikipedia, we find [the following is an abstract]
Denization is an obsolete or defunct process in […] Great Britain, the United Kingdom, and the British Empire, dating back to the 13th century, by which an alien (foreigner), through letters patent, became a denizen, thereby obtaining certain rights otherwise only normally enjoyed by the King's (or Queen's) subjects, including the right to hold land.
The status of denizen allowed a foreigner to purchase property, although a denizen could not inherit property. Sir William Blackstone wrote "A denizen is a kind of middle state, between an alien and a natural-born subject, and partakes of both." The denizen had limited political rights: he could vote, but could not be a member of parliament or hold any civil or military office of trust.
The term denizen may also refer to any national of a country, whether citizen or non-citizen, with a right to remain in and return to the country. In the United States, before they were made citizens by statute, unassimilated Native Americans, although born on U.S. soil, were not deemed to be citizens of the United States or any state, but of a domestic dependent nation contained within the United States but whose members are not even nationals of it, but denizens of it, with a right to remain in and return to their reserved territories.