How are physically obtained different rated voltage output -12/24/48 V- for a PMSG driven by a very small wind turbine rotor?
The power recovered from the wind is governed by the Betz limit:

$$P = \frac {1}{2} \rho A v^3$$

where: \$\rho\$ = Air density in kg/\$m^3\$, A = swept area of blades in \$m^2\$, and v = velocity in m/s. 

Maximum recoverable is 59% of this number.

Following graph shows this formula from: Wind Power - Wind Power Equation


So power varies with cube of wind speed.

The voltage induced or created is goverened by Faraday's Law:

$$V_{Ind} = N B \ell v$$

where N is number of turns, B = flux density in T, \$\ell\$ is length of a coil in m and v is velocity in m/s.

To get a rated voltage of 12V, 24V or 48V at the same rpm (velocity v) would typically mean stronger or more magnets or more windings and a slightly larger coil lenth.

So The rated wind speed is obviously fixed to a unique constant value.

But power generated and voltage generated depend on wind speed.  You size the generator based upon mean wind speed for the location, tower, storage.  

A generator sized for 12V and 400W at 300rpm will produce 400W at 12V if rotated at 300rpm, while a generator sized for 48V and 400W at 300rpm will produce 400W at 48V if rotated at 300rpm.  

Rotate either faster and power and voltage increase, but here is the rub, the most you can safely extract is 400W.  To extract this you need a three-phase ac to dc convertor capable of converting 5Vac to 68Vac to 13.6V DC for charging batteries (for the 12V PMSG).  Maximum generaged ac voltage goverened by maximum wind speed for wind turbine.

From WINDPMG



is the rotational speed of the rotor (rpm) always fixed for the rated power and the rated voltage related to a sort of electronic regulator?

Yes.  Below rated voltage and power, you extract whatever power you can.  The generators cut-in speed.  Above rated voltage and power, you extract rated power.  Extract more and you burn up the generator.

or is it possible to have different rotational speeds of the rotor for the same rated power using a variable speed control?

A gearing system would do the same thing, but a lot harder to control with wind gusts.