You have to go to Horses> Breed and put in a Breed Standard number  for each breed of horse. After that go to Horses> Manage> Breeding and you will see a little thing that says BV. The number under that will show for every horse and that is the Breed Variance.

Hi @LittleBird, in case you needed additional info or resources to what Rani above commented.

Breed Variance (BV) is based off of your horses Breed Standards (BS) and the breeds Ideal BS numbers.  So to find out a horses BV you first need to find the breed numbers that match the horse in question.    If you want to find out Breed Standard numbers on your own this Wiki Page has extra info on BS .   After reading through that, I would highly recommend using something like Google Sheets or Excel for tracking and finding out the Excellent BS Range.   After you find the ideal BS numbers you can follow what Rani provided above, manually find BV, or use one of the community made spreadsheets that you can import your CSV file into (If you need any more info or help with that feel free to ask!  If you have / are in the HoFa discord, there is a pinned Google Sheet in Help you can use).    

If you are not someone who wants to go through and find your own BS numbers for breeds, things like This BS Calculator made by Walrus have Spoiler Tabs with breed ideals on it that they have gathered.  

BV on its own is how far off from the ideal breed numbers your horse is.   So like a 5 star horse has a BV of 0-12 (this is helpful for making sure you are doing the math right!   If you get 16, but your horse is 5 stars, than you know your math or numbers are wrong)

Example

Say your ideal numbers are:  15, 90, 32, 80, 77, 63

Say your horses BS numbers are:  8, 87, 31, 79, 75, 48

Your horses BV would be 29 (3 stars [25-36]) because that is the sum of the differences between the numbers.

8 to 15 is 7, 87 to 90 is 3, 31 to 32 is 1, 79 to 80 is 1, 75 to 77 is 2, 48 to 63 is 15

7 + 3 + 1 + 1 + 2 + 15 = 29

I know thats a lot, I hope that maybe helped a lil bit, you are always welcome to reach out to me in game or on discord (FaminesRose in the server too) for any help or questions :)