For now, I found only the way of getting output exactly what you have mentioned using CASE so you can try and use it:

SELECT
id,
keywords = (
CASE WHEN kw1 IS NOT NULL THEN kw1 + CASE WHEN kw2 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw2 IS NOT NULL THEN kw2 + CASE WHEN kw3 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw3 IS NOT NULL THEN kw3 + CASE WHEN kw4 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw4 IS NOT NULL THEN kw4 + CASE WHEN kw5 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw5 IS NOT NULL THEN kw5 ELSE '' END)
FROM @tbl

OUTPUT:

id  keywords
-----------------------------------------
1   innocence;graphic novel;cartooncomics
2   tattoos;comics
3   music;cartoon;adventure;film

For now, I found only the way of getting output exactly what you have mentioned using CASE so you can try and use it:

SELECT
id,
keywords = (
CASE WHEN kw1 IS NOT NULL THEN kw1 + CASE WHEN COALESCE(kw2,kw3,kw4,kw5) IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw2 IS NOT NULL THEN kw2 + CASE WHEN COALESCE(kw3,kw4,kw5) IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw3 IS NOT NULL THEN kw3 + CASE WHEN COALESCE(kw4,kw5) IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw4 IS NOT NULL THEN kw4 + CASE WHEN kw5 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw5 IS NOT NULL THEN kw5 ELSE '' END)
FROM @tbl

OUTPUT:

id  keywords
-----------------------------------------
1   innocence;graphic novel;cartoon;comics
2   tattoos;comics
3   music;cartoon;adventure;film

For now, I found only the way of using CASE you can use if it seems suitable to your requirement:

SELECT
id,
keywords = (
CASE WHEN kw1 IS NOT NULL THEN kw1 + CASE WHEN kw2 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw2 IS NOT NULL THEN kw2 + CASE WHEN kw3 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw3 IS NOT NULL THEN kw3 + CASE WHEN kw4 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw4 IS NOT NULL THEN kw4 + CASE WHEN kw5 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw5 IS NOT NULL THEN kw5 ELSE '' END)
FROM @tbl

OUTPUT:

id  keywords
-----------------------------------------
1   innocence;graphic novel;cartooncomics
2   tattoos;comics
3   music;cartoon;adventure;film

For now, I found only the way of getting output exactly what you have mentioned using CASE so you can try and use it:

SELECT
id,
keywords = (
CASE WHEN kw1 IS NOT NULL THEN kw1 + CASE WHEN kw2 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw2 IS NOT NULL THEN kw2 + CASE WHEN kw3 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw3 IS NOT NULL THEN kw3 + CASE WHEN kw4 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw4 IS NOT NULL THEN kw4 + CASE WHEN kw5 IS NOT NULL THEN ';' ELSE '' END ELSE '' END +
CASE WHEN kw5 IS NOT NULL THEN kw5 ELSE '' END)
FROM @tbl

OUTPUT:

id  keywords
-----------------------------------------
1   innocence;graphic novel;cartooncomics
2   tattoos;comics
3   music;cartoon;adventure;film