Kevala kumbhaka does not require inhalation or exhalation and is considered the final stage of spiritual union, or samadhi. Kevala kumbhaka is not simply holding the breath between inhale and exhale. It is considered holding the prana completely separate of the movements of inhales or exhales, and is an unprompted stop of breath that occurs within a samadhi state attained through pranayama.
Nadi Shodhana pranayama is the powerful method to make a body healthy and strong, to increase energy and consciousness levels, calms down the mind and prepare it for deep meditation.
Nadi Shodhana pranayama is the best way to feed the body fully with oxygen and carbon dioxide at the same time.