Yawning is what happens when you reflexively opes your mouth wide, inhale, and exhale again. Most yawning is caused by sleepiness or extreme boredom.
Use the word yawning for actual yawning — or to mean "wide open." A yawning chasm in the ground is a huge, wide gap, and a yawning hole in a movie's plot is the obvious space where some details are missing, making the resulting story seem unlikely and unbelievable. A yawning child, on the other hand, is probably just ready for her nap.