LAWFUL RESIDENT: If you have been a resident in Kenya for a period of SEVEN years immediately preceding the date of application for Permanent Residence or Citizenship and have been so resident under authority of an entry permit or exemption, you are eligible for obtaining either Permanent Residency in Kenya or Citizenship.
MARRIAGE: If you have been married to a citizen of Kenya for a period of at least SEVEN years, you are also eligible for Permanent Residency in Kenya or Citizenship.
The differences:
As a Permanent Resident, you cannot;
Own a Kenyan Passport
Own a Kenyan ID
Vote
Own Agricultural land
An applicant for Citizenship is required to;
Be able to understand and speak Kiswahili or local dialect (basic)
Have adequate knowledge of Kenya and of duties and rights of citizens as contained in the Kenya Citizenship and Immigration Act
Understand the nature of the application
(The above 3 requirements will most likely have to be demonstrated before a Citizenship Advisory Committee)