Okay - What kind of servo are you talking about?
Most of the time on this forum a servo is referring to a hobby servo like used in RC aircraft and such.
If you are referring to a servo motor as used for motion control, it is a different animal.
Both Stepper Motors and Servo Motors can be purchased with a driver. For a stepper they generally take one of two input formats. Most common is Step/Direction and the other option is DirPos/DirNeg. Servo Motors have come in two styles - DC Brush type and more recently Brushless. The older DC Brush type motors had an analog input, typically -10V - +10V with the polarity controlling direction and voltage controlling speed. Newer Brushless controllers generally have two modes of operation - Step/Direction or Analog. Using the Step/Direction control option the Brushless Servo can be used in the same manner as a stepper motor.
Differences - A stepper motor has a limit on speed. As the stepper runs faster the torque falls. A stepper can be used with low ratio gearboxes/belt drives. A servo motor on the other hand has a better torque curve and is able to be run faster. 3000 - 5000 RPM is typical. Some motors are rated at 3000 RPM, but lightly loaded will perform nicely t 5000 RPM. Some Brushless drives have a selection of 2 motors - one design for speed and another design for more torque and less top speed. Brushless motors can also be configured as stand-alone drives where they are either configured to run at a specific RPM or a combinatin of inputs can be on/off and those patterns can select specific speeds. Because the brushless motor has a built in encoder the drive is able to control the RPM very closely in this mode.
Given a choice - If I could afford them I would rather use Brushless Servos, but they generally cost more, as much as 2 times, but they are also available in much higher power ratings. Brushless servos are generally rated in WATTS. 30, 50, 100 watt units are generally available to run on single phase 120VA and 200, 400, 800 watt units are available to run on 240VAC single or three phase.
PWM from an Arduino is probably not of high enough resolution to run a servo in analog mode. Though a Arduino could control a servo through the serial port.