PSW means Post Study Work and PSW Visa in UK is valid only on international student.
The Rule is in the following:
You will get 2 more year extension Visa if you are going to complete your graduation/postgraduate/Research onwards.
UK government giving 2 year PSW to the international students so
that they could secure a job there and if your salary would be around
£30000 then you could get 3 more year stay in UK and after that
according to UK Law’s you will get a permanent Residence or their
Passport very easily