Let us understand the meaning of the word 'Krishna'.
In the Vedas, the meaning of the word "Krishna" is Satchidanand Brahman (the absolute divinity full of bliss, knowledge, and eternity).
The one who attracts and entices everybody's heart (mind), the Supreme Soul, is Krishna. Even Lord Brahma, Lord Vishnu, and Lord Shankar (Shiva), are attracted to Shree Krishna. He attracts his own Self! Is it not astonishing?
The Supreme God, Shree Krishna has three main forms - Brahman, Paramatma and Bhagavan. Just like the three forms of water - steam, ice, and water itself (liquid). If we decrease the temperature of water it becomes ice, and if we increase the temperature, it becomes steam. Even though we have three forms, they are fundamentally the same. Similarly, there is only one God who manifests himself in three different forms.
Let us now understand the meaning and qualities of these three forms of God.
Brahman - This refers to the all-pervading form of God. Two main powers that are revealed in this form. The first is self-protection and the second is Swaroopanand (personal bliss). The other powers are present but are not manifest. Another important aspect of Brahman is that he is formless.
Paramatma - This refers to that form of God that resides in everyone's heart. The Paramatma also manifests in the personal form as Vishnu, and various avatars such as Narsingh and Varaha. Many more powers are manifest in the Paramatma form, than in the Brahman form.
Bhagavan - God reveals all his powers in this form. This is the form of Shree Krishna, in which he manifests his divine Names, Form, Pastimes, Abodes, and Associates.