Which pilot license is required in the US to engage in glider towing?
A private pilot certificate is the minimum required.  In addition you need the following experience and endorsements to be current:

CFR, part 61, section 61.69, outlines the requirements for towing privileges.
You must:


  1) Hold at least a private pilot certificate with the appropriate
  category rating;
  
  2) Have logged a minimum of 100 hours as pilot in command in the same
  category aircraft used for towing;
  
  3) Have a logbook endorsement from an authorized instructor certifying
  you have received ground and flight training in gliders or unpowered
  ultralight vehicles, and is proficient in the areas listed in part
  61.69(a)(3)(i)(ii)(iii) and (iv);
  
  4) Have a logbook endorsement from a pilot that already meets the
  requirements of part 61.69 (c) and (d), who has accompanied the pilot
  on three flights which has certified them to have accomplished at
  least three flights in an aircraft while towing a glider or unpowered
  ultralight vehicle, or while simulating towing flight procedures; and
  In the preceding 12 months has performed three actual or simulated
  tows accompanied by a qualified pilot or has been towed for three
  flights in a glider or unpowered ultralight vehicle. In accordance
  with part 61.52, pilots towing under an ultralight exemption may
  credit experience obtained in ultralight vehicles for their towing
  experience and endorsements.


Added per comment on question above:

Yes, a private pilot can be compensated for glider towing.

https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/data/interps/2010/umphres%20-%20(2010)%20legal%20interpretation.pdf

Source: https://www.faa.gov/licenses_certificates/airmen_certification/sport_pilot/media/towing_req.pdf