A dhadi jatha is two dhad players and a sarangi. They sing vaars, ballads of martyrdom, and the form is institutionally tied to the Akal Takht, where Guru Hargobind Sahib Ji is held to have first had dhadis perform.
In 2020 dhadis were barred from performing at the Takht after criticising the SGPC, which tells you the form is still doing the job it was created for.
Kavishri is the harder and rarer form: unaccompanied, rhythmic, declaimed rather than sung. Karnail Singh Paras, born 28 June 1916, died 28 February 2009, held the Sharomani Kavishar Award for 1980-81, cut around twenty records after a first All India Radio broadcast in 1954, and wrote for the Maan brothers.
The scholarly work is Hardial Thuhi's, in the Journal of Punjab Studies. Most of the rest of it is in Punjabi and has never been digitised.
The vaar is a heroic form and the dhadi's material is battle: Chamkaur, Muktsar, the eighteenth century, and in the twentieth century the Ghadar and the Akali morchas.
Dhadi jathas have been employed by the SGPC and by political parties, and the form has been political for as long as it has existed. That is why the 2020 bar mattered.
Recordings exist on cassette and on regional labels and almost none of it has been catalogued. The performers of the mid twentieth century are mostly unrecorded.