Arduino Driver can I use only PNP?
When connecting together several circuits that have different supply
voltages, common practice is to make them all have a common negative
supply rail. This is called “common ground”. As explained in Majenko’s
answer, you would need an NPN transistor for that.

If you want to use a PNP instead, then you could forget the common
practice and do everything “backwads”: if you connect together the positive
ends of the power supplies, you will have a “common VCC”
instead of a common ground. Relative to the Arduino ground, your
9&nbsp;V battery will have one pole at +5&nbsp;V and the other at
−4&nbsp;V.

The wiring is the following:


connect the emitter to +5&nbsp;V (Arduino +5 and battery +)
connect the base to an Arduino output through a resistor
connect the collector to the motor
connect the other end of the motor to the battery −
put a free wheeling diode in parallel with the motor.


Edit: Below is an ASCII art rendering of the circuit. Beware that
the motor is ON when the Arduino output is LOW.

+-----------+
|         5V|-----------------+-------------------+
|           |                 |                   |
|  Arduino  |                 |                   |
|           |    ,-----.  B |&lt; E                 ___ +
|        D13|----|     |----|    PNP              _
+-----------+    `-----'    |\                   ___
                              |C                  _
                              |                  ___
                              +---------+         _
                              |         |        ___ 9 V batt.
                              |         |         _
                             ---        |        ___
                              ^       motor       _
                             / \        |        ___
                             ---        |         _  −
                              |         |         |
                              |         |         |
                              +---------+---------+