When you enter the US on your immigrant visa, you are immediately a US permanent resident (a.k.a. green card holder), and you immediately have all the rights of a permanent resident including work and travel. The immigrant visa, stamped with an entry stamp upon entry, automatically serves as an I-551 (temporary proof of permanent resident status) for 1 year after the date of entry. So you can immediately work and/or travel using this immigrant-visa-turned-I-551 without waiting for your green card. You can start work before receiving the actual SS card, as long as you have applied for it (and I believe the immigrant visa application automatically applies for one for you; if they didn't you can apply for one yourself at the SSA office after entry).
As to whether a company can "hire" you, well, what do you mean by "hire"? Any company can sign you up for a future job even if you have no work authorization (or are not in the US) at the time -- you just need to have work authorization at the time you actually start the job (either applied by the company or by yourself). In this case, you don't need to company to get work authorization for you; if you are allowed into the US on your immigrant visa, you will immediately be authorized to work.
Thank you for the replies. I have spoken to recruiters and potential employers and they tell me to speak to them again when I can work in the US. Once I get my approval, I do not technically have the right to work as I would need to enter the US. However, practically I would since I would need to enter to start working and I would have the right to work as soon as I enter. I don't want to lie in an interview, but I do want to have a job before going.