A street is a basic paved traffic link within an urban area;
An avenue was originally grander, wider and often lined with trees or other flora. But the distinction has changed a bit with property developers call new roads “avenues” to make a more grandiose impression.
A boulevard is usually a widened, multi-lane arterial street with landscaping between the kerbs and pavements on either side. But like Avenue, there is a bit of exaggeration.
Broadly, a street will always be a street and a Avenue or Boulevard may still be just very slightly bigger streets!