Puedes probar con
SELECT i.name as name, v1.value as value_1, v2.value as value_2
FROM item i
INNER JOIN item_value iv ON iv.item = i.id
INNER JOIN property p ON iv.property = p.id
LEFT JOIN value v1 ON p.name = 'prop1' AND v1.id = iv.value
LEFT JOIn value v2 ON p.name = 'prop2' AND v2.id = iv.value
