Leaving the PWM thing aside for the moment, there is something wrong with your numbers. These numbers should mesh and yours don't. Heres why:
1) Kt = 1.351 Kv
2) Watts = RPM * in-oz / 1351
3) There are 746 Watts per Horsepower
A Kv of 15 RPM / 10V would mean the motor would have a no-load RPM of 180 at 120VDC.
From (1) it would make its Kt = 20.2 in-oz / Amp. At 10A its torque would be 202 in-oz.
From (2), its power would be 202 * 180 /1351 Watts or 27W
From (3), 1/3 HP = 249 Watts. 27W is not even close.
Please check your numbers. One of them is very badly off; the above is a reality check.