Unlike the full face mask, the nasal pillow only covers the nose, leaving your lips free and uncovered (if the full face mask reminds you of a SCUBA diver, think of the nasal pillow as a snorkel). But just because your mouth is outside of the mask doesn't mean it should be open during the night. Using a nasal pillow with a chinstrap combination can often get you a better seal. With fewer points to adjust, that's fewer chances for something to slip out of place. Invest in a high-quality chinstrap from RespShop to keep your jaw secure, reducing the chance of dry mouth.
Want more protection from dry mouth? Add a humidifier to your therapy kit. Together, all these different elements can help you achieve an effective and restful night's sleep.
Note: If you find it difficult to breath with a nasal pillow, or think you have issues relating to the pillow, you may need a full face mask. Consult with your doctor.