A yogi is one who is constantly striving to unite with Atman. S/he understands that the world is an illusory projection and is of zero value. S/he wants the Truth, the Reality. Everything s/he does is geared towards getting there. S/he does not get involved in anything, not even the spiritual path.
As against this, a tapasvi is one who is on the path of Bhakti Yoga but is lost in it. A jnani is pursuing knowledge but is attached to the path. A karmi does yeoman service but does it as an end in itself. The spiritual paths of Karma, Bhakti and Jnana Yogas are so exhilarating that there is a tendency to get stuck in them and lose sight of the goal.
A yogi maintains the goal in mind constantly and understands that the paths are only means to the end. They are also part of the world. They have importance only to the extent that they help attain Enlightenment. They are also finite. The goal is to get to the Infinite. The yogi is objective right from the beginning and maintains focus on the goal. S/he does not allow the world, however enchanting it may be, to detract from the goal of Atman.
The Isavasya Upanisad says that one who pursues avidya, the world, is in blinding darkness. But one who revels in vidya, the spiritual path, is in greater darkness! So beware of this impediment.
Among all yogis, one who pursues Karma, Bhakti and Jnana Yogas with sraddha is the best. Sraddha is a combination of emotional surrender and intellectual conviction. It is the consistency with which one strives to attain the goal and does not let go until the end is achieved. When intellectual focus and emotional yearning suffuse your spiritual practice you overcome the bulk of desires and become withdrawn and meditative. You are absorbed in the thought of Atman.
Such a yogi is considered to be most united. Effortlessly s/he attains Nirvana, Enlightenment.