Regards, Best regards and Kind regards don't have exactly the same meaning, but there are not a big differences between them. Each of them is prefered in different situations. It depends on who is the recipient of email or letter, for example friend, customer, employer, colleague etc.
- Regards - means that you have respect for the recipient (but not a close relationship with him or her)
- Best regards - is a semiformal, friendly but respectful closing for emails or letters. It can be used for both personal and professional correspondence.
- Kind regards - is a little more formal variation of Best regards. It is appropriate to use it in business correspondence, because it is considered to be more professional.