Below are restrictions currently in place for visa issue to people wishing to travel to Iran with certain passports :

US Passport :

Tourist visas : Not possible for independent travel. Travelers need to be travel with escort of an authorized guide on a pre-arranged program supervised by the visa sponsor.
The application processing time is 6-8 weeks. 
The MFA currently asks for some background information including work and study history and the applicants’ social media activity descriptions. 

Visa on Arrival : not possible

British & Canadian Passport :

Tourist visas : Should independent travel (only possible for pilgrimage or family visits) not approved , travelers need to travel with escort of an authorized guide on a pre-arranged program supervised by the visa sponsor. The application processing time is 6-8 weeks.

The MFA currently asks for some background information including work and study history and the applicants’ social media activity descriptions. 

If you are a Muslim, you may be able to apply for a pilgrimage visa which waives the guide requirement.

Visa on Arrival : not possible

Afghan & Bangladeshi Passports :

Tourist & Business visas : Not possible with travel agent support – Must refer to consulates directly. Visa on Arrival : not possible

Indian, Pakistani, Jordanian, Iraqi, Colombian and Somalia passports :

Visa on Arrival : not possible

Israeli Passport :

Issue of all Iranian visa (all types) is not possible