Q:

What knot can I use to connect to an eye bolt without losing tension?

So I have a rope that goes through an eyebolt, like in the image. I pull from B so it's as tight as it can be. B is under the eyebolt.
What knot can I use with this rope so I don't lose any of the tightness after pulling it?
Thanks.

A:

100% Trucker's hitch. It's the only knot I'd ever use in the situation you described:
..so I don't lose any of the tightness after pulling it.
From the linked page, above. Emphasis mine.
The Trucker's Hitch (also called the Lorry Knot) is a self binding knot. It's most common use is for tying loads to secure them to a fixed point. For example to secure a canoe to a car top, a tarp to a trailer, or any application where a very tight rope is needed. This knot has many advantages. It is non-jamming, can be tied anywhere in the standing part of a line, and even under the most severe tension, these knots remain easy to untie.
1) You must first tie a quick release loop above the tie-down point. To do this, create a small bight with the running end, leaving plenty of tag to work with.

2) When this is complete, pass the running end around or through the tie-down point then pass it through the quick release loop.

3) Pull down on the running end to tighten.

4) Secure the knot with two half-hitches.

5) Pull tight.

You can also find several videos on YouTube demonstrating how to tie the knot.
Learning this knot is a game-changer.

A:

Use a Midshipman's hitch (also called Taut-Line Hitch)
This is typically used to secure an awning or tent line to a peg, and it is adjustable.  
For your application, make the loop part large and loose.  Then pull on B, near the eyelet to tighten the load, then slide the knot down until everything is tight.
See:  http://www.animatedknots.com/midshipmans/index.php

