Karma yoga is done to overcome the selfish nature and seva is done to enhance the selfless nature. The action is the same but the orientation and intention of the action is different.
What is the definition of karma yoga? Sri Krishna says in the Bhagavad Gita (2:48):
Yogasthah kuru karmaani sangam tyaktvaa dhananjaya; Siddhyasiddhyoh samo bhootvaa samatvam yoga uchyat.
Perform action, O Arjuna, being steadfast in yoga, abandoning attachment and balanced in success and failure. Evenness of mind is the highest yoga.
To maintain balance in success and failure is the highest yoga. Successes and failures are decided by one’s interactions, performances and actions in life. Any action, anything that you do, if you can maintain your equipoise then it becomes karma yoga and it develops immunity from the effect of karma.
Whatever I do I do it always for the first time and the last time. Then it is clear, then it is finished and gone. It does not carry with me. That has to be the attitude and then karma yoga leads you to immunity from actions. As long as you are not immune to your actions karma yoga will always create
more conflict and confusion in the mind. You have to become immune to that by changing your own attitude, and this takes you beyond the selfish personality.
Karma yoga is meant to develop immunity and a positive attitude. Karma yoga is meant to realize the importance and beauty of karma. There is no need to shun and avoid any kind of karma but to realize the beauty and necessity of every karma and to be totally into it creatively.
Seva enhances the selfless quality in life when you can overcome your own limitations. Seva is something that you do even at home, when you sacrifice your time, your sleep, your food to serve somebody. If your son or daughter is sick at night you will be totally aware and alert as to the needs and you may not even sleep that night. You will be beside the bed, observing your offspring.
That sacrifice is in seva. Sacrifice comes in seva when you believe that you are looking after your own. For others it is difficult to sacrifice but for oneself one can make the effort.
That is the difference between karma yoga and seva.
—5 October 2014, Ganga Darshan, Munger