Yes, you need trekking permit. It’s not covered in your visa; once in Kathmandu it has to be taken out separately which are issued by the Department of Immigration Office in Kathmandu and Pokhara. The criteria or requirement for the Trekking permit are Visa, trekking fee and two colored passport sized photos. However, Issuance of permits is categorized based into different areas and regions. For instance for trekking areas such as Everest, the Annapurna and Langtang one require permit altogether different from the permit issued if you are climbing mountain or peak, it falls under a whole different category, and will require a different permit. Note, however, that a trekking permit does not allow you to go anywhere in the country either.