This type of letter is used when there is not necessarily currently a job opening. There has been no official notice of a job but you wish to apply for employment.
Your opening or first paragraph should introduce who you are and why you are writing. Tell them what position you are qualified for, why you are interested in their company and how you came to know of them.
Your second paragraph should expound on your interest in their company, what makes you right for the job including any pertinent experience, and any other details that might interest them in employing you.
Your third paragraph should announce the CV you have enclosed. Tell them about any particular areas on the CV that might help you get an interview.
Finally, your closing paragraph should state your available times to interview. You should also show great interest in their company in particular and briefly mention again why it would be a good match for both of you.
Ask them to consider you for future job openings then end your letter gracefully.