Swara is Sanskrit word, meaning sound or note. It is also a continuous flow of air through one nostril. Yoga means union, so Swara yoga is a science which is realization of cosmic consciousness through control and manipulation of breath.
Swara Yoga is science which is a complete study, observations, control and manipulation of breath or Swara. Pranayama is only related to control of breath in various ways. In swara yoga, you will find association of breath in relation to activities of sun, moon, various seasons, physical and mental conditions of individuals etc. So Swara Yoga is more comprehensive in theory and practices related to breath.