What is black’s compensation in the main line of the Caro Kann? After move 10 in the main line of Caro Kann, black has already moved its light-squared bishop four times, while white manages to develop quite a bit.

What exactly is blacks compensation for the lack of development?

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1. e4 c6 2. d4 d5 3. Nc3 dxe4 4. Nxe4 Bf5 5. Ng3 Bg6 6. h4 h6 7. Nf3 Nd7 8. h5 Bh7 9. Bd3 Bxd3 10. Qxd3
At a basic level, what we want from an opening and a defense, is: we want to be fighting for the center squares (1), we're trying to develop our pieces and get good squares for them (2), and thirdly, we want to have a safe king (3). This is as modest a expectation as one can have for a good opening.

Now roughly speaking, there are two types of defenses (say e.g. against 1.e4): First type: we attack the pawn on e4, like the Scandinavian, the French, and the Caro-Kann,..., in other words, any defense where we immediately set out to immediately attack the pawn on e4 is the first type of defense. The second type of defense is to leave the e4 pawn alone and basically, black's saying I'll go and take my fair share of the center, typical example is the Sicilian (1...c5 first dark-square claim). 

In diagram you've posted, we see a typical Caro-Kann structure where black's trumps are:


we have successfully managed to exchange one of white's central pawns (e4), which was the primary goal of our defense.
There's definite prospect for challenging white's 2nd central pawn (d4) with a timely c5 or e5 advance in the middlegame. And the d5 square is a nice post for a knight.
In case white aims for c4 to control d5, then d4 being in a semi-open file, becomes a bit of a soft spot which black can target, and the extra expansion on the queenside by white means their queenside castling is less likely or at least less safe. 


So it's fair to say expectation (1) is ticked off. Now onto (2):


Black is clearly behind in development: white has 5 developed pieces (pawns and nonpawns), black has 3, moreover, white's ready to castle kingside and 1 tempo away from queenside castling whereas black's kingside castle is at least 3 tempi away. 
However, thanks to our successful neutralisation of the e4 pawn, the central struggle has quieted down, and our path to finishing development is rather clear (and resolved for the most part), namely: e6-Nf6-Be7-O-O.
It's not just that we are finishing development, but we are in fact achieving decent piece activity: most importantly, the knight on f6 which plays both a very important defensive role for the king, and provides good central control. The d5 square could in certain cases be a good outpost for either the knight or even sometimes the queen. In case of the former, then f6 square becomes free for our d7 knight to jump to.
The knight on d7 is not poorly placed as it is ideally placed to eye both c5 and e5, the two pawn advances black will be aiming for next in order to challenge white's d4 pawn (dark-square control).
Same holds for the bishop, which more importantly, is not hemmed-in inside our own structure.


These points tick off expectation (2). Now onto (3):


As said, we have no trouble in castling kingside, where the king will be well protected by our minor pieces (knight on f6, bishop e7 and knight on d7 which can go to f8 if need be).
Thanks to white's over-advancement of the h pawn all the way to h5, their prospect of exploiting our hook h6 on the kingside is not so easy anymore. Ideally, to exploit the h6 advance when pushing against the king, is to have the h4-g4 setup. All to say, tempo-wise, white is sufficiently away from amassing a major threat on our kingside. 
Last but not least, white has no means of decoying our minor piece defense on the kingside, and the somewhat weakened light squares are less of a concern since the respective bishops have been traded off.


These points all suggest that we have in fact a solid structure and our kingside castling remains safe, so (3) is ticked off as well. 



In conclusion, we have seen why the structures that emerge from Caro-Kann (in particular for the posted variation) are solid and more importantly, we see that black's opening has successfully achieved our 3 basic expectations, so we can give ourselves a tap on the back and claim to have played a successful defense. 

Extra bonus aspects about the position:


The queen can easily swing over to the queenside b6 or a5 and create threats, in general you want to be inducing additional pawn advances from white on the queenside to make it more difficult for them to castle safely on the queenside. So manoeuvres like Qa5 and Qb6 are much needed!
The d-file is semi-open for black, which gives us a pressure point and rook activity. 
The c5 advance (and sometimes e5) should be a part of your middlegame strategy in order to neutralise white's remaining central pawn and dark square control. 
The h5 pawn can sometimes be a real weakness, that you can easily target.
A queenside expansion is almost always perfectly valid.
Last but not least, I highly encourage you to study Caro-Kann games for inspiration. For example, look into the games of Vassily Ivanchuk, Jovanka Houska, Yasser Seirawan or Aleksandra Goryachkina, who've produced many instructive Caro-Kann games.