What is the difference between the spiritual master and the Supreme Personality of Godhead.

“The example of Lord Krishna’s being the Supreme Personality of Godhead is appropriate in regard to understanding the spiritual master. The spiritual master is called sevaka-bhagavan, the servitor
Personality of Godhead, and Krishna is called sevya-bhagavan, the Supreme Personality of Godhead who is to be worshiped. The spiritual master is the worshiper God, whereas the Supreme Personality of Godhead, Krishna, is the worshipable God. This is the difference between the spiritual master and the Supreme Personality of Godhead.

Another point: Bhagavad-gita, which constitutes the instructions of the Supreme Personality of Godhead, is presented by the spiritual master as it is, without deviation. Therefore the Absolute Truth is present in the spiritual master. The Supreme Personality of Godhead gives real knowledge to the entire world, and the spiritual master, as the representative of the Supreme Godhead, carries the message throughout the world. Therefore, on the absolute platform, there is no difference between the spiritual master and the Supreme Personality of Godhead. If someone considers the Supreme Personality–Krishna or Lord Ramacandra–to be an ordinary human being, this does not mean that the Lord becomes an ordinary human being. Similarly, if the family members of the spiritual master, who is the bona fide representative of the Supreme Personality of Godhead, consider the spiritual master an ordinary human being, this does not mean that he becomes an ordinary human being. The spiritual master is as good as the Supreme Personality of Godhead, and therefore one who is very serious about spiritual advancement must regard the spiritual master in this way. Even a slight deviation from this understanding can create disaster in the disciple’s Vedic studies and austerities.”

Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Seventh Canto, Chapter 15 – Text 27