Khannnas basically hail from Panjab. Part of Khatri community which is also a part of Thatris folower of Lord "sri Ram" of Hindu mythology of Asia continent.
Initially the Vedic Arya Kshatriyas were divided in two clans - the Suryavanshis and the Chandravanshis. Later some families of both the clans became followers of the Rishi Angira and became known as the Agnikul. The Chandravanshi Raja Nahush's son Yayati and his son Yaddu's descendants became the Yaduvanshis.
The power of the Kshatriyas declined over the years and conflict arose between the Brahmins and the Kshatriyas. The Kshatriyas lost their dominant position.They lost their states and finally came and settled in Punjab. They started calling themselves Khatris. The Suryavanshis adopted another name of Surya that is Meher and became Mehras, Mehrotras and Malhotras and the Chandravanshis adopted the name Kapoors.
The Agnivansh split up. Some became brahmins and called themselves Agnihotris and the rest were called the 'Khand' family (the 'part' family). Later the Khand family came to be known as Khanna's. Since they were from a 'split' clan - they were called the 'half' clan. The so called 'Dhai Ghars' (two and a half families) are the Kapoors, Malhotras and the Khannas.
(Source: Khatri Samaj Sanskar avam Prathayaen, Vishveshwar Nath Mehrotra published by the All India Khatri Mahasabha.)
Our family of Khanna's comes from Kot Maldeo now in Pakistan. A large part of the clan from that village has now settled at Lahli, District Rohtak, Haryana. India