Some useful definitions of diction, tone and voice from Mary Oliver’s A Poetry Handbook.
Diction:
“‘Watch your language!’ you may say to someone who has just expressed himself with the help of slang or an expletive. What you really mean is ‘Watch your diction!’ Diction means word choice."
Tone:
“The overall effect of the diction of a piece of writing, in addition to other elements, such as choice of subject, imagery, design of the poem, etc., is called tone.”
Voice:
“The term voice is used to identify the agency or agent who is speaking through the poem, apart from those passages that are actual dialogue. This voice, or speaker of the poem, is often called the persona.”