DC controlled servo drive means that the motor driver is coupled with the control system using an analog signal + /-10V. When 0V is given, the engine is not running. When the voltage is e.g. +10 V, the motor rotates at full speed, and when the voltage is -10V, it rotates at maximum speed in the opposite direction. For other voltages the rotational speed is proportional to the preset voltage (which is supervised by the speed controller in the motor driver on the basis of the feedback from the tachometer generator). How does the CNC control system know what voltage has to be given? This requires the position feedback which is carried out using linear scales or encoders. The position given is subtracted from the measured position and the control system gives the motor driver the signal which is proportional to this difference. This is the so-called control system for the offset which is proportional to the speed. The higher the speed, the greater the deviation of the shape. This is an anachronism.
The wording "DC servo drive" may also indicate that a direct current (DC) motor has been used for the construction of the servo drive, which does not necessarily mean that the speed is given using DC. Servo drives of this type can be controlled by discrete CLK/DIR signals. The driver in such a servo drive tries to control the engine so that the position offset approached zero regardless of the rotational speed of the engine. This is a bit younger solution but outdated as well and goes out of use.
Nowadays, when it comes to servo drives, only brushless motors with electronic commutation with an encoder on the shaft are applied. There are two versions of such servo drives: BLDC - brushless DC motors and PMAC - synchronous alternating current (AC) motors with permanent magnets.