Unfortunately your parents would not be able to settle in the UK from a visit visa. In order to be eligible to settle in the UK you must have spent 5 years continuous residency on a 'working' visa or 10 years continuous residency on some other routes.
There are currently two routes available to meet the 5 years residence requirement. One of your parents could find a job in the UK and apply for a Tier 2 (General) visa with their partner as a dependant. In order to be eligible for a Tier 2 visa the applicant must have a confirmed offer of employment with an employer who holds a valid sponsor licence and is willing to issue you with a Certificate of Sponsorship.
They must also have £945 in personal savings for themselves plus an additional £630 for their dependant to prove you can support themselves and this must have been held in their account for at least 90 days prior to the application.
Alternatively, if they are sponsored by an A Rated sponsor it is possible for them to certify your maintenance. They must also prove their knowledge of English is at least CEFR level B1.
There is also a minimum salary requirement attached to each job role, each individual salary is detailed with the SOC codes in Appendix J, please see the link below:
The other route would be Tier 1 (Entrepreneur). The requirements of the Entrepreneur category are that an applicant must be able to show access to £200k in cash funds, and satisfy the English Language requirement. There is also a maintenance requirement where a certain level of savings must have been maintained for 90 days before making the application. It is mandatory to satisfy the Genuine Entrepreneur test and it is vital therefore to present a well prepared business plan as part of the process, which must be clearly related to the skills and experience of the applicant.