As kind is a countable noun (used in this sense), it must have an article when singular.
So it is either people of different kinds, or people of a different kind.
However whilst the former is used to refer to two or more kinds of people e.g. The anti-EU voters comprise peoples of many different kinds, the latter is used to apply to a single kind of people, who are different to some reference group, or groups e.g. UKIP voters are people of an entirely different kind to all others.