By bus it will take about 12 hrs 

By train 13 hrs 

By air 1-2 hrs

The best option would be by bus because of money time ratio.

Hi,

how to reach kasol from delhi : -

Delhi To Kasol by Air

Train passengers can travel from Delhi to Chandigarh via train. Exit the railway in Chandigarh and catch a HRTC bus bound for Kullu or Manali. All buses heading to Manali or Kullu make a stop in Bhuntar. So, tell the bus driver ahead of time that you want to be dropped off in Bhuntar. Tourists must take another bus or cab from Bhutar to Kasol.

Delhi To Kasol by Rail

There are no direct trains from Delhi to Kasol, thus people travelling by rail must first take a train to Bhuntar. There is no direct rail from Delhi to Bhuntar. One can take the train to Chandigarh and then drive the rest of the way. The nearest railway station to Kasol is Joginder Nagar, which is approximately 143 kilometres away. Book a cab or a bus and forget about wondering how to get from Delhi to Kasol by train.

Delhi To Kasol by Road

There is no direct bus service from Delhi to Kasol; you must alight at Bhuntar bus station. Traveling by bus from Delhi to Kasol takes approximately 12 to 13 hours. Private operators' buses from Delhi to Kasol depart at Majnu ka Tila in Delhi. While government buses run from Kashmiri Gate ISBT in Delhi to Bhuntar on a regular basis. You can take a cab or a bus from here to Kasol. The journey from Bhuntar to Kasol takes roughly 2 hours. Have a wonderful journey!

There are no direct trains from Delhi to Kasol, thus people travelling by rail must first take a train to Bhuntar. There is no direct rail from Delhi to Bhuntar. One can take the train to Chandigarh and then drive the rest of the way. The nearest railway station to Kasol is Joginder Nagar, which is approximately 143 kilometres away. 

If you are willing to travel to Kasol from Delhi by local transport so the best option for you is to take a bus from either Majnu Ka Tila /R.K Ashram or from Kashmiri Gate for Volvo and Ordinary bus respectively.

For more details you can contact/check the online portals of ISBT and HRTC.