The incident occurred in Kohat city's Peshawar Chowk in the morning.
Miscreants had planted a bomb near the bus stand, police said, adding the bodies and injured were rushed to hospital. The van was destroyed for which five kilogramme explosives were used.
It was the second attack on a passenger bus in the province. Earlier, seven people were killed in the previous blast that took place in Peshawar, according to Dawn online.
The province is witnessing what appears to be a blowback from the military operation in the tribal region of North Waziristan.
The provincial capital of Peshawar has also witnessed a sudden rise in attacks that have targeted police and military officials along with civilian interests.