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*.