Bhakta Prahlada is a devotee of Lord Vishnu, whereas Krishna is considered as a divine deity, but still both of them have lot of similarities in their life. Lord Krishna killed the wicked Kansa, whereas Prahlad had killed the wicked Holika, when she tried to kill him.
Lord Krishna had shown his Viswaroopa Darshan during the Kurukshetra war, and similarly, when once Lord Indra tried to attack Prahlada, Lord Narasimha had entered into the body of Bhakta Prahlada, and Prahlada had shown his Viswaroopa darshan to Lord Indra and to other Devas.
Prahalad in his previous birth was served under the court of Lord Brahma Deva, in the name of SanguKarna, and he is an incarnation of Lord Brahma. Similarly Lord Krishna is an incarnation of Lord Vishnu. Both of them were kings, and ruled their kingdom in a proper manner. They won the hearts of the people by giving a golden rule, and lived their life in the path of Dharma, and both of them have done lot of good deeds in their life. Their teachings are also based on devotion to the god, and doing service to the mankind.
Similar to Lord Krishna, who is having Sudarshana Chakra in his hand, Prahlada contains a divine sword which was granted as a gift by his beloved Lord Sri Narasimha, when he was sworn as a king. Both of them contains great spiritual powers, and utilized it only in case of necessity. Prahalada in his succeeding births took birth as Bahlika Raja, and was very much liked by Lord Krishna, though he fought on the side of the Kauravas. In this Kali Yuga, Prahlada took the avatars of Sri Vyasaraja and Sri Raghavendra, and in both the avatars, he had shown his true bhakti on Lord Krishna.
Still in some Narasimha Temples, the idol of Prahlada is considered as the Urchava idol of Narasimha, and it is taken out during the times of procession, and festival times. Still in some parts of North India, village people used to worship Bhakta Prahlada during the time of Holi festival, in order to get all kinds of prosperity in their life.