In the tradition, gurus are categorized in four groups. One is the vidwan guru, knowledgeable guru, the second is the yogi guru, the third is the tantric guru, and the fourth is the brahmanishtha guru, the enlightened guru.
The first category of guru is the vidwan or jnani guru. There are many jnanis: the intellectuals, the knowledgeable people. It is not necessary that they be a sadhaka, a practitioner, just as a university professor can train students to become a scientist but is not an Einstein himself. The teacher of Einstein was not Einstein. In the same manner, the knowledgeable guru is able to impart knowledge, but he is not the scientist who has gone through the process himself. He can tell you what is right and what is not, but it is not necessary for him to live those precepts. Usually, such gurus are public figures. They give lectures and programs, classes, seminars and conventions.
The second category is the yogi guru. A yogi guru is not a yoga teacher. You are a yoga teacher, but you are not a yogi guru. A yogi guru is a person who has achieved the highest state of yoga in life. What is the highest state of yoga in life? According to Patanjali, it is chitta vritti nirodhah. One who has been able to control the modifications of the mind, whose mind does not vascillate, who is one¬pointed, centred, balanced and peaceful, and emanates that. A yogi guru is going through a process of learning and teaching at the same time.
The third is the tantric guru, the siddha. He is able to experience the cosmic energy manifesting in the body. One who is able to connect the body with the cosmic energy is the tantric guru. He can invoke that energy in matter, he can manifest the unmanifest, he is able to control the Shiva and Shakti elements.
The fourth category is the brahmanishtha guru and there are very few such gurus. They are people in whom the energy of God is fully awakened.They are not called avataras, but they can be called small avataras, kanistha avatara. The brahmanishtha guru has nothing to do with the world, but forever remains in communion with God. He can see God all the time before him just as we can see each other. Ramakrishna Paramahamsa and Ramana Maharishi were the kind of people who belong to this category.Swami Sivananda was also a brahmanishtha guru. He was an enlightened person. He knew everything.
Once I asked Sri Swamiji, “Swami Sivanandaji has written so many books and given references to so many literatures and scriptures in his writings. When did he have time to read them? Did he ever read any book?” Sri Swamiji said, “No, he never read any book, but the title and content of every book was on the tip of his tongue. He would write without any crosses, without any changes or editing. What he used to write was final. He was a person of the calibre in whom the knowledge manifested spontaneously.” That is the brahmanishtha guru. One who does not need to read anything, but knows everything.

While Swami Sivananda was a brahmanishtha guru, Sri Swamiji was a tantric guru. He was able to explode the energies, invoke the shaktis and bring about a greater transformation just through the force of his presence, his being.
In fact, Sri Swamiji as a sadhaka moved from level to level. At first, when he was teaching yoga, he was a yogi guru. He became a tantric guru when he involved himself in the higher vedic sadhanas. And from 2003, he became a brahmanishtha guru, for that was when he became absolutely free from all his karmas. In fact, Sri Swamiji’s life helps us understand the concept of guru. I have seen Sri Swamiji and the association he had with his guru, Swami Sivananda – the dedication, devotion and faith that he had in his guru. Due to that conviction he progressed in life and evolved to the highest levels of cosmic realization. Many gurus only remain a knowledgeable or vidya guru, many only remain a tantric guru, but Sri Swamiji kept moving from one stage to another. In each stage of his life, his attainments are clearly defined: what he was when he was in Munger, what he became when he went to Rikhia, and what he became when he left Rikhia. Everyone has been witness to that transformation. However, these grades do not mean that one guru is better than the other. The differentiation made between gurus, however, is only for the sake of understanding the role of guru in our life. For a disciple no guru is higher or lower than the other. They are all at the same level. Don’t see the four categories as vertical attainments; see them as horizontal behaviors of different individuals.

From the book “Dharma of a disciple”, Swami Niranjanananda Saraswati