I know that the  M91/30 is long, but what are the differences in the M38 and M44?

Simply put, the M44 and the M38 are both carbine length.  The 44 has an attached bayonet.  The 44 was meant for front line work while the M38 was designed for support and rear echelon.  The 44 was derived from the 38.  The 38 will not take a bayonet.

The 44 was made by the USSR, Poland, Romania, and Hungary.  The Chinese made one they named the Type 53 (for 1953).  

You also have the 91/59 which is a 91/30 cut down to resemble a 38.  It retains the 91/30 rear sight which as been milled down to shorter ranges.  It will not take a bayo, either.

The 91/38 was a Czech rework of mostly Czarist M91s, cut down to 30 length.  Again, no bayo will fit nor is there the attachment as found on the M44.

Quoted: I know that the  M91/30 is long, but what are the differences in the M38 and M44?

A side folding bayonet on the M44.  M44's are also more recent with most built post WWII.

Many, if not most, of the Russian M44s were built during the war.  All the other countries made their post WWII.