Manufacturers use different wire winds to produce different KV results. KV stands for thousands ( K ) of rpms per volt.

So, a 1000KV motor will turn 1000 rpms when 1 volt is applied. It will turn 7000 rpms if 7 volts are applied.

Lower KV motors typically produce more torque so they are typically used with large propellers that will be turned slower. These are very popular on gliders, for example, where climb angle and climb rate is much more important than top speed.

Higher KV motors are typically used with smaller props for higher top speeds. Or they can be used with gear boxes to handle those big props.

You also match based on what battery you plan to use.

If you look here you will see that a given motor is offered in several KV ratings. They make suggestions as to which motor is best matched with which prop and which pattery packs.
http://www.maxxprod.com/mpi/mpi-262.html

Hope that helps.