The treatment of ARTt in both the poems is quite different.
In ODE ON A GRECIAN URN, the poet has depicted a frozen moment from the sylvan history of the greek civilization. Remember, the poet calls the grecian urn "attic shape" and "Fair attitude" adding that it "dost tease us out of thought/ As doth eternity." The poem explores the possibility of communication between the two worlds: the temporal and the eternal- reflected in the concept of 'the now' and 'the then'.
Moreover, the images on the urn depict a love that is far above "all breathing human passion that leaves a heart high-sorrowful and cloyed, a burning forehead , and a parching tongue." In the same way, Keats emphasizes that the trees shall never bid Spring adieu, nor shall the lady ever lose her beauty.Therefore, art in a way is superior than human life. The theme of the poem is permanence of art and impermanence of huma life.
The treatment of art in Tennyson's THE LADY OF SHALOTT is entirely different. The lady has heard a curse that she must not look down at Camelot. So she has to live a life of isolation; not to be a part of this human world. Hence, she weaves a magic web by night and day with colours gay. The shadows of the world appear before her on the magic mirror, and she continues to rave, imagining the life she is devoid of. The moment she watches Lancelot, she decides to leave her palace and moves to be a part of this world. She invites the curse to fall upon her and finally meets her death.
Many critics see the poem as an allegory which has deeper meanings. So long as one is sheltered away from the real world and is able to see the reflections only, it is a beautiful world. But the moment one faces reality one finds it to be harsh and cruel world. Perhaps, the poet wishes to say that creative artists should be more in touch with reality. Remember, Tennyson's works need more attentive reading and one can interpret several connotations.